Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Derivative [Line Items]
Equity Securities, FV-NI, Unrealized Gain (Loss) Restricted	$ 21	$ 0
Fair value of long term debt	9,500		$ 9,600
Fair Value, Inputs, Level 1 [Member]
Derivative [Line Items]
Fair value of long term debt	8,500		$ 8,600
Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Fair value of long term debt	1,000
Marketable Securities [Member]
Derivative [Line Items]
Unrealized Losses on Equity Securities	22
Deferred compensation plan assets [Member]
Derivative [Line Items]
Unrealized Losses on Equity Securities	$ 9