Stockholders' Equity - Narrative (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 29, 2016	Oct. 31, 2013
Class of Stock [Line Items]
Amount of common stock repurchases authorized				$ 1,000,000,000.0	$ 750,000,000.0
Value of shares repurchased during period	$ 361,194,000	$ 373,794,000	$ 302,606,000
Remaining amount available for future purchases of shares under approved repurchase program.	$ 333,300,000
Common Stock
Class of Stock [Line Items]
Repurchases of common stock (in shares)	6.9	7.0	4.5
Value of shares repurchased during period	$ 361,200,000	$ 373,800,000	$ 302,600,000