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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Mar. 31, 2020
Mar. 31, 2019
Sep. 30, 2019
Derivative [Line Items]          
Derivative liabilities $ 2,609   $ 2,609   $ 2,568
Derivative assets 65   65   97
Fair value of long term debt 7,000   7,000   7,600
Fair Value, Inputs, Level 1 [Member]          
Derivative [Line Items]          
Fair value of long term debt 6,800   6,800   7,400
Fair Value, Inputs, Level 2 [Member]          
Derivative [Line Items]          
Fair value of long term debt 200   200   $ 200
Marketable Securities [Member]          
Derivative [Line Items]          
Unrealized gains (losses) (32) $ 21 (22) $ (1)  
Marketable Securities [Member] | Restricted investments          
Derivative [Line Items]          
Unrealized gains (losses) (32) 20      
Deferred compensation plan assets [Member]          
Derivative [Line Items]          
Unrealized gains (losses) $ (10) $ 7 $ (7) $ (2)