Stockholders' Equity - Narrative (Details) - USD ($) shares in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Feb. 29, 2016
Class of Stock [Line Items]
Stock repurchase program, authorized amount						$ 1,000,000,000.0
Repurchase authorization increase	$ 416,700,000
Authorized and available for repurchase		$ 750,000,000		$ 750,000,000
Repurchases of common stock		(165,727,000)		(185,512,000)	$ (177,615,000)
Amortization expense from capitalized stock-based compensation		$ 5,800,000	$ 3,900,000	$ 11,400,000	$ 7,400,000
Common Stock [Member]
Class of Stock [Line Items]
Shares repurchased during period (in shares)		2.2		2.5