Fresh Start Accounting - Narrative (Details) - USD ($)
 2 Months Ended 10 Months Ended 12 Months Ended
Dec. 15, 2017
Dec. 15, 2017
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Dec. 16, 2017
Fresh-Start Adjustment [Line Items]            
Enterprise value $ 5,721,000,000 $ 5,721,000,000        
Repayment of debtor-in-possession financing 725,000,000   $ 0      
Debt maturing within one year     29,000,000      
Debt face amount     3,253,000,000      
Funding payment for Avaya represented employee pension plan     0      
Debt cancelled due to bankruptcy $ 5,721,000,000          
Class of warrant or right, number of securities called by each warrant or right (in shares) 5,645,200 5,645,200        
Repayment of long-term debt     22,000,000      
Payment to PBGC     0      
Establish Successor Goodwill $ 2,658,000,000 $ 2,658,000,000       $ 2,658,000,000
Discharge of Debt            
Fresh-Start Adjustment [Line Items]            
Debt maturing within one year 29,000,000 29,000,000        
Accounts payable (49,000,000) (49,000,000)        
Adjustments, accounts payable, reclassification of accrued bankruptcy related professional fees (50,000,000) (50,000,000)        
Adjustments, accounts payable, payment of bankruptcy professional fees (3,000,000) (3,000,000)        
Adjustments, accounts payable, reinstatement of contract cure costs 4,000,000 4,000,000        
Payroll and benefit obligations 23,000,000 23,000,000        
Fair value adjustment to Deferred revenue 79,000,000 79,000,000        
Deferred revenue 50,000,000 50,000,000        
Adjustments, other liabilities, deferred revenue 29,000,000 29,000,000        
Adjustments, restructuring reserve 7,000,000 7,000,000        
Business restructuring reserve 3,000,000 3,000,000        
Business restructuring reserve 4,000,000 4,000,000        
Adjustments, liabilities subject to compromise, pension obligation (295,000,000) (295,000,000)        
Funding payment for Avaya represented employee pension plan 49,000,000          
Other post-retirement obligations 212,000,000 212,000,000        
Change in other liabilities 233,000,000 233,000,000        
Payment to PBGC 340,000,000          
Property, plant and equipment, net 0 0        
Intangible assets, net 0 0        
Change in other assets 6,000,000 6,000,000        
Change in other current liabilities 65,000,000 65,000,000        
Revaluation of Assets            
Fresh-Start Adjustment [Line Items]            
Property, plant and equipment, net 116,000,000 116,000,000        
Intangible assets, net 3,137,000,000 3,137,000,000        
Change in other assets (27,000,000) (27,000,000)        
Revaluation of Liabilities            
Fresh-Start Adjustment [Line Items]            
Accounts payable 0 0        
Payroll and benefit obligations 0 0        
Fair value adjustment to Deferred revenue (235,000,000) (235,000,000)        
Deferred revenue (341,000,000) (341,000,000)        
Business restructuring reserve 0 0        
Business restructuring reserve 4,000,000 4,000,000        
Other post-retirement obligations 0 0        
Change in other liabilities (43,000,000) (43,000,000)        
Change in other current liabilities (3,000,000) (3,000,000)        
Minimum            
Fresh-Start Adjustment [Line Items]            
Enterprise value 5,100,000,000 5,100,000,000        
Maximum            
Fresh-Start Adjustment [Line Items]            
Enterprise value 7,100,000,000 7,100,000,000        
Term Loan            
Fresh-Start Adjustment [Line Items]            
Debt face amount     $ 2,903,000,000      
First Lien Debt            
Fresh-Start Adjustment [Line Items]            
Debt cancelled due to bankruptcy $ 4,281,000,000          
Common stock issued for debt (in shares) 99,300,000          
Common stock issued for Predecessor debt $ 1,509,000,000          
Repayment of long-term debt 2,061,000,000          
Second Lien Debt            
Fresh-Start Adjustment [Line Items]            
Debt cancelled due to bankruptcy $ 1,440,000,000          
Common stock issued for debt (in shares) 4,400,000          
Common stock issued for Predecessor debt $ 67,000,000          
Term Loan Credit Agreement due December 15, 2024 | Term Loan            
Fresh-Start Adjustment [Line Items]            
Debt face amount $ 2,925,000,000 2,925,000,000        
Debt term 7 years          
Term Loan Credit Agreement due December 15, 2024 | Term Loan | Discharge of Debt            
Fresh-Start Adjustment [Line Items]            
Debt face amount $ 2,925,000,000 2,925,000,000        
Revolving Credit Facility | ABL Credit Agreement | Line of Credit            
Fresh-Start Adjustment [Line Items]            
Debt term 5 years          
Line of credit facility, current borrowing capacity $ 300,000,000 300,000,000        
Predecessor            
Fresh-Start Adjustment [Line Items]            
Repayment of debtor-in-possession financing   725,000,000   $ 0 $ 0  
Debt maturing within one year       725,000,000    
Debt face amount       6,557,000,000    
Funding payment for Avaya represented employee pension plan   49,000,000   0 0  
Repayment of long-term debt   111,000,000   223,000,000 25,000,000  
Payment to PBGC   340,000,000   0 $ 0  
Common stock issued for Pension Benefit Guaranty Corporation 92,000,000 92,000,000        
Goodwill 3,541,000,000 3,541,000,000        
Predecessor | Term Loan            
Fresh-Start Adjustment [Line Items]            
Debt face amount       $ 0    
Warrants            
Fresh-Start Adjustment [Line Items]            
Fair value of warrant liability 17,000,000 $ 17,000,000        
Pension Plan | Avaya Inc. Pension Plan for Salaried Employees            
Fresh-Start Adjustment [Line Items]            
Payment to PBGC $ 340,000,000          
Common stock issued for PBGC (in shares) 6,100,000 6,100,000        
Common stock issued for Pension Benefit Guaranty Corporation $ 92,000,000 $ 92,000,000