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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Sep. 30, 2019
Derivative [Line Items]      
Derivative liabilities $ 2,611   $ 2,568
Derivative assets 106   97
Equity Securities, FV-NI, Unrealized Gain (Loss) Restricted 10 $ (21)  
Fair value of long term debt 7,600   7,600
Fair Value, Inputs, Level 1 [Member]      
Derivative [Line Items]      
Fair value of long term debt 7,400   7,400
Fair Value, Inputs, Level 2 [Member]      
Derivative [Line Items]      
Fair value of long term debt 200   $ 200
Marketable Securities [Member]      
Derivative [Line Items]      
Equity Securities, FV-NI, Unrealized Gain (Loss) 10 (22)  
Marketable Securities [Member] | Restricted investments      
Derivative [Line Items]      
Equity Securities, FV-NI, Unrealized Gain (Loss)   (21)  
Deferred compensation plan assets [Member]      
Derivative [Line Items]      
Equity Securities, FV-NI, Unrealized Gain (Loss) $ (3) $ (9)