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Fair Value Measurements - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Sep. 30, 2018
Derivative [Line Items]          
Equity Securities, FV-NI, Unrealized Gain (Loss) Restricted $ 9 $ 0      
Proceeds from Sale and Maturity of Marketable Securities       $ 3  
Marketable Securities, Realized Gain (Loss)       $ 2  
Fair value of long term debt 7,500   $ 7,500   $ 9,600
Fair Value, Inputs, Level 1 [Member]          
Derivative [Line Items]          
Fair value of long term debt 7,300   7,300   8,600
Fair Value, Inputs, Level 2 [Member]          
Derivative [Line Items]          
Fair value of long term debt 200   200   $ 1,000
Marketable Securities [Member]          
Derivative [Line Items]          
Equity Securities, FV-NI, Unrealized Gain (Loss) $ 9   8    
Deferred compensation plan assets [Member]          
Derivative [Line Items]          
Equity Securities, FV-NI, Unrealized Gain (Loss)     $ (2)