Financing Arrangements - Narrative (Details)
2 Months Ended 8 Months Ended 10 Months Ended 12 Months Ended
Jun. 18, 2018
Jun. 11, 2018
USD ($)
day
$ / shares
shares
Dec. 15, 2017
USD ($)
$ / shares
Jan. 19, 2017
USD ($)
Dec. 15, 2017
USD ($)
$ / shares
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
$ / shares
shares
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Debt Instrument [Line Items]                  
Debt cancelled due to bankruptcy     $ 5,721,000,000            
Debt face amount             $ 3,253,000,000    
Liability component of debt             3,253,000,000    
Purchase of convertible note bond hedge             $ 84,000,000    
Class of warrant or right, exercise price of warrants or rights (in USD per share) | $ / shares     $ 25.55   $ 25.55        
Weighted average contractual interest rate of debt             6.40%    
Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           $ 6,557,000,000   $ 6,557,000,000  
Purchase of convertible note bond hedge         $ 0     0 $ 0
Warrants issued (in shares) | shares             124,500,000    
Amortization of debt issuance costs and discounts               $ 61,000,000  
Interest expense not recorded         94,000,000 $ 316,000,000      
Weighted average contractual interest rate of debt           7.70%   7.70%  
Predecessor | Affiliated Entity                  
Debt Instrument [Line Items]                  
Liabilities subject to compromise, capital lease obligations and accrued interest           $ 10,000,000   $ 10,000,000  
First Lien Debt                  
Debt Instrument [Line Items]                  
Fair value of assets received by debt holders     $ 3,570,000,000   3,570,000,000        
Debt cancelled due to bankruptcy     4,281,000,000            
First Lien Debt | Senior secured term B-4 loans                  
Debt Instrument [Line Items]                  
Debt face amount             $ 0    
First Lien Debt | Senior secured term B-4 loans | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           1,000,000   1,000,000  
First Lien Debt | Senior secured term B-6 loans                  
Debt Instrument [Line Items]                  
Debt face amount             0    
First Lien Debt | Senior secured term B-6 loans | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           519,000,000   519,000,000  
First Lien Debt | Senior secured term B-3 loans                  
Debt Instrument [Line Items]                  
Debt face amount             0    
First Lien Debt | Senior secured term B-3 loans | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           594,000,000   594,000,000  
First Lien Debt | Senior secured term B-7 loans                  
Debt Instrument [Line Items]                  
Debt face amount             0    
First Lien Debt | Senior secured term B-7 loans | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           $ 2,012,000,000   $ 2,012,000,000  
First Lien Debt | 7% senior secured notes                  
Debt Instrument [Line Items]                  
Debt face amount             $ 0    
Interest rate, stated percentage           7.00% 7.00% 7.00%  
First Lien Debt | 7% senior secured notes | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           $ 982,000,000   $ 982,000,000  
First Lien Debt | 9% senior secured notes                  
Debt Instrument [Line Items]                  
Debt face amount             $ 0    
Interest rate, stated percentage           9.00% 9.00% 9.00%  
First Lien Debt | 9% senior secured notes | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           $ 284,000,000   $ 284,000,000  
Second Lien Debt                  
Debt Instrument [Line Items]                  
Fair value of assets received by debt holders     84,000,000   84,000,000        
Debt cancelled due to bankruptcy     1,440,000,000            
Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Debt face amount             $ 0    
Interest rate, stated percentage           10.50% 10.50% 10.50%  
Second Lien Debt | 10.50% senior secured notes | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           $ 1,440,000,000   $ 1,440,000,000  
Term Loan                  
Debt Instrument [Line Items]                  
Debt face amount             $ 2,903,000,000    
Interest expense on debt             154,000,000    
Term Loan | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           0   0  
Term Loan | Term Loan Credit Agreement due December 15, 2024                  
Debt Instrument [Line Items]                  
Debt face amount     $ 2,925,000,000   $ 2,925,000,000        
Term Loan | Revolving Credit Facility | LIBOR Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     4.75%   4.75%        
Term Loan | Revolving Credit Facility | LIBOR Loans | Floor | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 0.00%   1.00%            
Term Loan | Revolving Credit Facility | LIBOR Loans | Federal Funds Rate | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 0.50%                
Term Loan | Revolving Credit Facility | LIBOR Loans | Alternative Base Rate | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 3.25%                
Term Loan | Revolving Credit Facility | LIBOR Loans | LIBOR | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate 4.25%                
Term Loan | Revolving Credit Facility | ABR Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     3.75%   3.75%        
Term Loan | Revolving Credit Facility | ABR Loans | Federal Funds Rate                  
Debt Instrument [Line Items]                  
Basis spread on variable rate     0.50%            
Line of Credit | Revolving Credit Facility | ABL Credit Agreement                  
Debt Instrument [Line Items]                  
Line of credit facility, current borrowing capacity     $ 300,000,000   $ 300,000,000        
Minimum fixed charge coverage ratio     1   1        
Minimum net borrowing availability     $ 25,000,000   $ 25,000,000        
Percent of total borrowing base     10.00%   10.00%        
Line of Credit | Revolving Credit Facility | Base Rate Loans                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | Base Rate Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     0.75%   0.75%        
Line of Credit | Revolving Credit Facility | Base Rate Loans | Federal Funds Rate                  
Debt Instrument [Line Items]                  
Basis spread on variable rate     0.50%            
Line of Credit | Revolving Credit Facility | LIBOR Loans Denominated In US Dollars                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | LIBOR Loans Denominated In US Dollars | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     1.75%   1.75%        
Line of Credit | Revolving Credit Facility | Canadian Prime Rate Loans                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | Canadian Prime Rate Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     0.75%   0.75%        
Line of Credit | Revolving Credit Facility | CDOR Loans                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | CDOR Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     1.75%   1.75%        
Line of Credit | Revolving Credit Facility | LIBOR Loans Denominated In Sterling                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | LIBOR Loans Denominated In Sterling | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     1.75%   1.75%        
Line of Credit | Revolving Credit Facility | EURIBOR Loans Denominated In Euro                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | EURIBOR Loans Denominated In Euro | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     1.75%   1.75%        
Line of Credit | Revolving Credit Facility | Overnight LIBOR Loans                  
Debt Instrument [Line Items]                  
Step up or step down percentage     0.25%   0.25%        
Line of Credit | Revolving Credit Facility | Overnight LIBOR Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage     1.75%   1.75%        
Line of Credit | Letter of Credit | ABL Credit Agreement                  
Debt Instrument [Line Items]                  
Letters of credit, maximum amount     $ 150,000,000   $ 150,000,000        
Letters of credit outstanding             46,000,000    
Letter of credit, remaining borrowing capacity             153,000,000    
Convertible Notes                  
Debt Instrument [Line Items]                  
Debt face amount   $ 350,000,000         $ 350,000,000    
Interest rate, stated percentage   2.25%         2.25%    
Interest expense on debt             $ 7,000,000    
Proceeds from convertible debt   $ 314,000,000              
Conversion ratio (shares per principal)   36.0295              
Conversion price (in usd per share) | $ / shares   $ 27.76              
Redemption price   100.00%              
Liability component of debt   $ 258,000,000              
Equity component of debt   92,000,000              
Unamortized issuance costs   10,000,000         $ 7,000,000    
Effective interest rate             9.20%    
Convertible Notes | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           0   0  
DIP Credit Agreement                  
Debt Instrument [Line Items]                  
Debt face amount             $ 0    
DIP Credit Agreement | Predecessor                  
Debt Instrument [Line Items]                  
Debt face amount           725,000,000   $ 725,000,000  
DIP Credit Agreement | Base Rate Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage       6.50%          
DIP Credit Agreement | Base Rate Loans | Federal Funds Rate                  
Debt Instrument [Line Items]                  
Basis spread on variable rate       0.50%       0.50%  
DIP Credit Agreement | Base Rate Loans | Eurocurrency Rate | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate       2.00%          
DIP Credit Agreement | DIP Credit Agreement due January 19, 2018 | Predecessor                  
Debt Instrument [Line Items]                  
Letters of credit outstanding       $ 150,000,000          
Liability component of debt       $ 725,000,000          
Cash collateralization percent, US dollar       101.50%          
Cash collateralization percent, alternative currencies       103.00%          
DIP Credit Agreement | Eurocurrency Loans | Minimum                  
Debt Instrument [Line Items]                  
Interest rate, stated percentage       7.50%          
DIP Credit Agreement | Eurocurrency Loans | Eurocurrency Rate | Minimum                  
Debt Instrument [Line Items]                  
Basis spread on variable rate       1.00%          
Secured Debt | Senior secured term B-4 loans | Floor                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               5.25%  
Basis for effective rate, floor               2.25%  
Secured Debt | Senior secured term B-4 loans | LIBOR                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               6.25%  
Basis for effective rate, floor               1.25%  
Secured Debt | Senior secured term B-4 loans | Effective Yield Payable Minus Effective Yield Payable                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               50.00%  
Secured Debt | Senior secured term B-6 loans | Floor                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               4.50%  
Basis for effective rate, floor               2.00%  
Secured Debt | Senior secured term B-6 loans | LIBOR                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               5.50%  
Basis for effective rate, floor               1.00%  
Secured Debt | Senior secured term B-6 loans | Effective Yield Payable Minus Effective Yield Payable                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               50.00%  
Secured Debt | Senior secured term B-3 loans | Floor                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               3.50%  
Secured Debt | Senior secured term B-3 loans | LIBOR                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               4.50%  
Secured Debt | Senior secured term B-7 loans | Floor                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               4.25%  
Basis for effective rate, floor               2.00%  
Secured Debt | Senior secured term B-7 loans | LIBOR                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               5.25%  
Basis for effective rate, floor               1.00%  
Secured Debt | Senior secured term B-7 loans | Effective Yield Payable Minus Effective Yield Payable                  
Debt Instrument [Line Items]                  
Basis spread on variable rate               50.00%  
Over-Allotment Option | Convertible Notes                  
Debt Instrument [Line Items]                  
Debt face amount   $ 50,000,000              
Conversion Circumstance One | Convertible Notes                  
Debt Instrument [Line Items]                  
Trading days | day   20              
Consecutive trading days | day   30              
Conversion price threshold   130.00%              
Conversion Circumstance Two | Convertible Notes                  
Debt Instrument [Line Items]                  
Consecutive trading days | day   5              
Conversion price threshold   98.00%              
Business day threshold   5 days              
Bond Hedge                  
Debt Instrument [Line Items]                  
Amount of shares hedged (in shares) | shares   12,600,000              
Purchase of convertible note bond hedge   $ 84,000,000              
Bond hedge strike price (in usd per share) | $ / shares   $ 27.76              
Bond Hedge and Call Spread Warrants                  
Debt Instrument [Line Items]                  
Conversion price (in usd per share) | $ / shares             $ 37.3625    
Call Spread                  
Debt Instrument [Line Items]                  
Warrants issued (in shares) | shares   12,600,000              
Value of warrants   $ 58,000,000              
Class of warrant or right, exercise price of warrants or rights (in USD per share) | $ / shares   $ 37.3625              
Other liabilities                  
Debt Instrument [Line Items]                  
Capital lease obligations             $ 31,000,000    
Other liabilities | Predecessor                  
Debt Instrument [Line Items]                  
Capital lease obligations           14,000,000   $ 14,000,000  
Liabilities subject to compromise, capital lease obligations and accrued interest           12,000,000   12,000,000  
Avaya Private Cloud Services Business | Other liabilities                  
Debt Instrument [Line Items]                  
Capital lease obligations             $ 26,000,000    
Avaya Private Cloud Services Business | Other liabilities | Predecessor                  
Debt Instrument [Line Items]                  
Capital lease obligations           $ 24,000,000   $ 24,000,000  
Before April 1, 2017 | First Lien Debt | 7% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               101.75%  
Before April 1, 2017 | First Lien Debt | 9% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               102.25%  
On Or After April 1, 2017 | First Lien Debt | 7% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
On Or After April 1, 2017 | First Lien Debt | 9% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
Upon occurance of specific kinds of changes of control | First Lien Debt | 7% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               101.00%  
Upon occurance of specific kinds of changes of control | First Lien Debt | 9% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               101.00%  
Upon occurance of specific kinds of changes of control | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               101.00%  
Under certain assets sales | First Lien Debt | 7% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
Under certain assets sales | First Lien Debt | 9% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
Under certain assets sales | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
On March 1, 2017 | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               107.875%  
On March 1, 2018 | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               105.25%  
On March 1, 2019 | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               102.625%  
On or after March 1, 2020 | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%  
Prior to March 1, 2017 plus a make whole premium | Second Lien Debt | 10.50% senior secured notes                  
Debt Instrument [Line Items]                  
Redemption price               100.00%