Debt (Details) - USD ($)	3 Months Ended	6 Months Ended
	Mar. 31, 2015	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Repayments of convertible senior notes		$ 0	$ (12,650,000)
Treasury Stock, Shares		3,763,868		3,763,868
Treasury Stock, Value		$ 316,240,000		$ 316,240,000
Payments in connection with deduction of outstanding warrants		0	242,117,000
Credit Facility [Domain]
Line of Credit Facility, Maximum Borrowing Capacity	$ 750,000,000
Line of Credit Facility, Expiration Period	5 years
Line of Credit Facility, Fair Value of Amount Outstanding		$ 0
Convertible senior notes
Debt Conversion			12,700,000
Repayments of convertible senior notes			$ (12,700,000)
Stock Issued During Period, Shares, New Issues			118,822
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments			$ 47,100,000
Treasury Stock, Shares			118,808
Treasury Stock, Value			$ 10,000,000
November 2015 Agreement [Member]
Limit price per share for warrant holders to close out hedge position			$ 535.00
Payments in connection with deduction of outstanding warrants			$ 135,200,000
Reduction of number of warrants			360,406
February 2016 Agreement [Member]
Payments in connection with deduction of outstanding warrants			$ 106,900,000
Reduction of number of warrants			403,665
Limit price per share for warrant holders to close out hedge position pursuant to the agreement			$ 375.00