Operating Segments - Summarized Financial Information of Operating Segments (Details) - USD ($)
$ in Millions
 1 Months Ended 2 Months Ended 3 Months Ended 10 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 15, 2017
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Dec. 16, 2017
Segment Reporting Information                          
REVENUE $ 148   $ 735 $ 692 $ 672         $ 2,247      
GROSS PROFIT 78   390 352 323         1,143      
OPERATING EXPENSES                          
Selling, general and administrative                   888      
Research and development                   172      
Amortization of intangible assets                   127      
Impairment of indefinite-lived intangible assets                   0      
Goodwill impairment                   0 $ 52 $ 442  
Restructuring charges, net                   81      
TOTAL OPERATING EXPENSES                   1,268      
OPERATING (LOSS) INCOME $ 2   11 $ (49) $ (89)         (125)      
INTEREST EXPENSE, OTHER INCOME (EXPENSE), NET AND REORGANIZATION ITEMS, NET                   (134)      
(LOSS) INCOME BEFORE INCOME TAXES                   (259)      
Total Assets     7,679             7,679     $ 7,609
Products & Solutions                          
OPERATING EXPENSES                          
Goodwill impairment                     0    
Services                          
OPERATING EXPENSES                          
Goodwill impairment                     52    
Operating Segments | Products & Solutions                          
Segment Reporting Information                          
REVENUE                   1,052      
GROSS PROFIT                   696      
OPERATING EXPENSES                          
Total Assets     1,336             1,336      
Operating Segments | Avaya Networking                          
Segment Reporting Information                          
REVENUE                   0      
GROSS PROFIT                   0      
Operating Segments | Services                          
Segment Reporting Information                          
REVENUE                   1,401      
GROSS PROFIT                   843      
OPERATING EXPENSES                          
Total Assets     1,509             1,509      
Unallocated                          
Segment Reporting Information                          
REVENUE                   (206)      
GROSS PROFIT                   (396)      
OPERATING EXPENSES                          
Total Assets     $ 4,834             $ 4,834      
Predecessor                          
Segment Reporting Information                          
REVENUE   $ 604       $ 790 $ 803 $ 804 $ 875   3,272 3,702  
GROSS PROFIT   362       496 493 484 535   2,008 2,258  
OPERATING EXPENSES                          
Selling, general and administrative   264                 1,261 1,401  
Research and development   38                 225 273  
Amortization of intangible assets   10                 204 226  
Impairment of indefinite-lived intangible assets   0                 65 100  
Goodwill impairment   0         52       52 442  
Restructuring charges, net   14                 30 105  
TOTAL OPERATING EXPENSES   326                 1,837 2,547  
OPERATING (LOSS) INCOME   36       69 $ (43) $ 75 $ 70   171 (289)  
INTEREST EXPENSE, OTHER INCOME (EXPENSE), NET AND REORGANIZATION ITEMS, NET   3,400                 (369) (430)  
(LOSS) INCOME BEFORE INCOME TAXES   3,436                 (198) (719)  
Total Assets           5,898         5,898    
Predecessor | Operating Segments | Products & Solutions                          
Segment Reporting Information                          
REVENUE   253                 1,297 1,536  
GROSS PROFIT   169                 890 1,047  
OPERATING EXPENSES                          
Total Assets           1,133         1,133    
Predecessor | Operating Segments | Avaya Networking                          
Segment Reporting Information                          
REVENUE   0                 140 219  
GROSS PROFIT   0                 48 80  
Predecessor | Operating Segments | Services                          
Segment Reporting Information                          
REVENUE   351                 1,835 1,947  
GROSS PROFIT   196                 1,091 1,160  
OPERATING EXPENSES                          
Total Assets           2,499         2,499    
Predecessor | Unallocated                          
Segment Reporting Information                          
REVENUE   0                 0 0  
GROSS PROFIT   $ (3)                 (21) $ (29)  
OPERATING EXPENSES                          
Total Assets           $ 2,266         $ 2,266