Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2016
Debt Conversion		$ 1,700,000
Repayments of convertible senior notes	$ 0	$ (1,739,000)
Stock Issued During Period, Shares, New Issues		16,774
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
Treasury Stock, Shares		16,768
Treasury Stock, Value		$ 1,400,000
Additional Paid-in Capital [Member]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments		$ 6,700,000
November 2015 Warrant Agreement
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 Warrant Agreement
Limit price per share for warrant holders to close out hedge position		$ 375.00
Payments in connection with deduction of outstanding warrants		$ 106,900,000
Reduction of number of warrants		403,665