Debt (Details) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended
	Feb. 28, 2015	Nov. 30, 2014	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Debt Conversion					$ 12,700,000	$ 144,000,000
Repayments of convertible senior notes					$ (12,650,000)	$ (144,001,000)
Stock Issued During Period, Shares, New Issues					118,822	1,399,069
Treasury Stock, Shares					3,761,628		3,642,820
Gains (Losses) on Extinguishment of Debt						$ 16,900,000
Treasury Stock, Value					$ 316,052,000		$ 306,069,000
Payments in connection with deduction of outstanding warrants					(242,117,000)	$ (124,531,000)
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			1,399,056		118,808	1,399,056
Treasury Stock, Value			$ 117,500,000		$ 10,000,000	$ 117,500,000
Convertible Debt					200,000		$ 11,200,000
Additional Paid-in Capital [Member]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments					$ 47,100,000	$ 694,700,000
Warrant Transactions [Member]
Warrant outstanding balance					1,345,027
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
Maximum reduction of warrants held by warrant holder					975,142
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
November 2014 Warrant Agreement
Limit price per share for warrant holders to close out hedge position		$ 397.75
Payments in connection with deduction of outstanding warrants	$ (124,031,000)
Adjustments to Additional Paid in Capital, Warrant Issued				$ 23,317,114
Reduction of number of warrants			416,480