Convertible Senior Notes - Narrative (Details) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Debt issued	$ 690,000,000	$ 690,000,000	$ 690,000,000
Threshold trading days exceeding price	20 days
Threshold consecutive trading days exceeding price	30 days
Threshold greater than percentage of stock price trigger	130.00%
Threshold trading days not exceeding price	5 days
Threshold consecutive trading days not exceeding price	5 days
Principal amount per conversion	$ 1,000
Threshold less than percentage of stock price trigger	98.00%
Conversion rate (in shares)	11.1651
Conversion price (in dollars per share)	$ 89.56
Potential conversion shares of convertible debt (in shares)	7.7
Fair value of convertible senior notes		$ 697,000,000
Closing stock price (in dollars per share)		$ 65.04
Repurchases of common stock	$ 62,000,000	$ 361,194,000	$ 373,794,000	$ 302,606,000
Payments for purchase of convertible note hedge and warrant transactions	23,300,000
Payments for note hedge transactions	$ 101,300,000
Note hedge shares outstanding (in shares)	7.7
Warrants outstanding (in shares)	7.7
Warrant strike price (in dollars per share)	$ 104.49
Proceeds from sale of warrants	$ 78,000,000
Effective interest rate	3.20%