Convertible Senior Notes - Narrative (Details) $ / shares in Units, shares in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2014 USD ($) d $ / shares shares	Mar. 31, 2018 USD ($) $ / shares	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Debt issued	$ 690,000,000	$ 690,000,000		$ 690,000,000
Threshold trading days exceeding price | d	20
Threshold consecutive trading days exceeding price | d	30
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	0.0111651
Conversion price (in dollars per share) | $ / shares	$ 89.56
Potential conversion shares of convertible debt (in shares) | shares	7.7
Fair value of convertible senior notes		$ 712,900,000
Closing price of common stock (in dollars per share) | $ / shares		$ 70.98
Repurchases of common stock	$ 62,000,000	$ 19,785,000	$ 72,467,000
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) | shares	7.7
Warrants outstanding (in shares) | shares	7.7
Warrant strike price (in dollars per share) | $ / shares	$ 104.49
Proceeds from sale of warrants	$ 78,000,000
Effective interest rate	3.20%