Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended				9 Months Ended	12 Months Ended
	Sep. 30, 2018	Aug. 31, 2018	Jun. 30, 2018	Feb. 28, 2018	Sep. 30, 2018	Sep. 30, 2018	Jul. 31, 2018	Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Authorized share-repurchase program				$ 3,000			$ 4,000	$ 2,500
Total Shares Delivered					38.3
Amount of share repurchase program completed					$ 2,441	$ 3,500
Accelerated Stock Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Amount	$ 1,941		$ 1,441	$ 500	$ 1,941	$ 1,941
Average Price per Share (in usd per share)			$ 65.28	$ 58.82
Initial Shares Delivered			19.1	7.0	26.1
Additional Shares Delivered			3.0	1.5	4.5
Total Shares Delivered			22.1	8.5	30.6
Open-market Stock Purchases [Member]
Equity, Class of Treasury Stock [Line Items]
Amount	$ 250	$ 250			$ 500
Average Price per Share (in usd per share)	$ 63.11	$ 66.14
Total Shares Delivered	3.9	3.8			7.7