Stockholders' Equity - Repurchase of Common Stock (Detail) - USD ($)	1 Months Ended		4 Months Ended	12 Months Ended
	Feb. 20, 2018	Sep. 30, 2017	Aug. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 30, 2017	May 31, 2017	Mar. 31, 2017	Jul. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Treasury stock value				$ 1,302,100,000	$ 217,000,000	$ 802,000,000
Treasury stock value				$ 2,700,600,000	$ 1,478,100,000
Common stock in treasury, shares				61,485,423	34,156,355	29,612,337					14,151,759
July 2015 Repurchase Program
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Stock repurchase program, authorized amount										$ 1,500,000,000.0
Stock repurchase program, additional authorized amount									$ 1,500,000,000.0
Number of common stock shares repurchased				27,320,816
Treasury stock value				$ 1,200,000,000
Common stock repurchase (price per share)				$ 45.44
Common Stock Repurchase Set Two | Share Trading Plan
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Number of common stock shares repurchased					4,680,313
Treasury stock value					$ 217,000,000
Common stock repurchase (price per share)					$ 46.36
Common Stock Repurchase Set One | Share Trading Plan
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Number of common stock shares repurchased				8,169,390
Treasury stock value				$ 370,500,000
Common stock repurchase (price per share)				$ 45.35
Accelerated Share Repurchase Agreement
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Stock repurchase program, authorized amount		$ 400,000,000.0		$ 400,000,000.0			$ 400,000,000.0
Number of common stock shares repurchased		9,147,415	2,914,955
Treasury stock value				$ 320,000,000
Common stock repurchase (price per share)		$ 43.73	$ 44.47
Treasury stock value								$ 150,000,000
Share repurchase program, remaining authorized repurchase amount			$ 129,600,000
Common stock in treasury, shares				7,089,056
Subsequent Event | Accelerated Share Repurchase Agreement
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Common stock repurchase (price per share)	$ 48.14
Common stock in treasury, shares	8,308,692