Debt (Details) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2011
Debt Instrument [Line Items]
Treasury Stock, Value	$ 316,240,000				$ 316,240,000	$ 316,240,000
Net carrying amount of the liability component [Abstract]
Treasury Stock, Shares	3,763,868				3,763,868	3,763,868
Warrants [Abstract]
Adjustments to Additional Paid in Capital, Other						$ (643,365,000)	$ (449,456,000)
Payments for Repurchase of Warrants					$ 0	643,365,000	573,487,000
Warrants and Rights Outstanding	$ 0					0
Maximum amount of warrant payable							(23,317,000)
Convertible Debt [Abstract]
Repayments of Convertible Debt					0	$ 12,894,000	$ 166,467,000
Stock Issued During Period, Shares, New Issues						121,058	1,625,113
Loss on Extinguishment of Debt							$ 18,861,000
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments						$ 47,789,000	819,657,000
Line of Credit Facility [Abstract]
Treasury Stock, Value, Acquired, Cost Method						$ 0	$ 0
November 2016 Agreement [Member] [Member]
Warrants [Abstract]
Payments for Repurchase of Warrants	401,200,000
Warrant
Warrants [Abstract]
Reduction of number of warrants		360,406	115,970				898,547
Payments for Repurchase of Warrants		$ 135,300,000	$ 50,000,000				$ 399,500,000
February 2016 Agreement [Member]
Warrants [Abstract]
Reduction of number of warrants						403,665
Payments for Repurchase of Warrants						$ 106,900,000
Convertible senior notes
Debt Instrument [Line Items]
Debt Instrument, Face Amount								$ 400,000,000.0
Interest rate, stated percentage								1.875%
Initial conversion price (in dollars per share)								$ 84.02
Credit Facility [Domain]
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity				$ 750,000,000
Line of Credit Facility, Expiration Period				5 years
Available increase to the credit facility				$ 250,000,000
Line of Credit Facility, Current Borrowing Capacity	$ 100,000,000					100,000,000
Line of Credit Facility, Fair Value of Amount Outstanding					$ 0
Additional Paid-in Capital
Warrants [Abstract]
Adjustments to Additional Paid in Capital, Other						(643,365,000)	(449,456,000)
Maximum amount of warrant payable							(23,317,000)
Convertible Debt [Abstract]
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments						47,789,000	819,657,000
Line of Credit Facility [Abstract]
Treasury Stock, Value, Acquired, Cost Method						10,171,000	136,539,000
Treasury Stock
Line of Credit Facility [Abstract]
Treasury Stock, Value, Acquired, Cost Method						$ (10,171,000)	$ (136,539,000)
Treasury Stock, Shares, Acquired						121,048	1,625,088