Convertible Senior Notes - Narrative (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	1 Months Ended	12 Months Ended
	Feb. 28, 2014 D	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Debt issued	$ 690,000,000	$ 690,000,000
Conversion rate (in shares)	11.1651
Principal amount per conversion	1,000
Conversion price (in dollars per share)	$ 89.56
Threshold trading days exceeding price	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	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)	7.7
Fair value of convertible senior notes		720,900,000
Closing stock price (in dollars per share)		$ 62.96
Repurchases of common stock	62,000,000	268,647,000	160,419,000	141,468,000
Payments for purchase of convertible note hedge and warrant transactions	23,300,000
Payments for note hedge transactions	101,300,000	101,292,000	0	0
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	$ 77,970,000	$ 0	$ 0
Effective interest rate	3.20%