Convertible Senior Notes - Narrative (Details) $ / shares in Units, shares in Millions	1 Months Ended	6 Months Ended
	Feb. 28, 2014 USD ($) d $ / shares shares	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Debt issued	$ 690,000,000	$ 690,000,000		$ 690,000,000
Conversion rate (in shares)	11.1651
Principal amount per conversion	$ 1,000
Conversion price (in dollars per share) | $ / shares	$ 89.56
Threshold trading days exceeding price | d	20
Threshold consecutive trading days exceeding price	30 days
Threshold greater than percentage of stock price trigger	130.00%
Threshold trading days not exceeding price | d	5
Threshold consecutive trading days not exceeding price	5 days
Threshold less than percentage of stock price trigger	98.00%
Potential conversion shares of convertible debt (in shares) | shares	7.7
Fair value of convertible senior notes		$ 720,900,000
Closing price of common stock (in dollars per share) | $ / shares		$ 69.82
Repurchases of common stock	$ 62,000,000	$ 126,068,000	$ 187,491,000
Payments for purchase of convertible note hedge and warrant transactions	23,300,000
Payments for note hedge transactions	$ 101,300,000	0	101,292,000
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	$ 0	$ 77,970,000
Effective interest rate	3.20%