Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended					3 Months Ended		9 Months Ended		12 Months Ended
	Oct. 19, 2017	Sep. 30, 2017	Aug. 31, 2017	May 31, 2017	Mar. 31, 2017	Feb. 28, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 11, 2015	Oct. 23, 2014	Jul. 25, 2013
Stockholders Equity [Line Items]
Dividends declared per common share							$ 0.34	$ 0.31	$ 0.99	$ 0.90
Cost of treasury stock		$ 2,000,000,000					$ 2,000,000,000		$ 2,000,000,000			$ 500,000,000	$ 1,000,000,000	$ 500,000,000
Cumulative shares repurchased		36,588,112					36,588,112		36,588,112		33,837,526
Average price of common stock purchased		$ 39.25	$ 41.77	$ 40.65	$ 45.15	$ 45.30			$ 45.88		$ 46.16
Payments For Repurchase Of Common Stock		$ 1,679,000,000					$ 1,679,000,000		$ 1,679,000,000
Maximum
Stockholders Equity [Line Items]
Cost of treasury stock		$ 2,000,000,000					$ 2,000,000,000		$ 2,000,000,000
Subsequent Event
Stockholders Equity [Line Items]
Declaration Date	Oct. 19, 2017
Dividends declared per common share	$ 0.34
Cash dividend, date to be Paid	Dec. 05, 2017
Cash dividend, recorded date	Nov. 21, 2017