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Convertible Notes (Interest Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 17, 2017	Aug. 16, 2013
Interest expense related to notes
Amortization of Debt Issuance Costs and Discounts	$ 7,243	$ 6,854	$ 9,243
Additional paid in capital	$ 1,270,426	$ 1,261,142
Convertible senior notes \| 1.375% convertible notes due 2023
Debt instrument
Debt Instrument, Interest Rate, Stated Percentage	1.375%	1.375%	1.375%	1.375%
Convertible senior notes \| 1.125% Convertible Senior Notes due 2018
Debt instrument
Debt Instrument, Interest Rate, Stated Percentage			1.125%		1.125%
Convertible senior notes
Interest expense related to notes
Total interest expense on convertible notes	$ 9,615	$ 9,226	$ 11,991
Convertible senior notes \| 1.375% convertible notes due 2023
Interest expense related to notes
Coupon interest	2,372	2,372	2,372
Amortization of Debt Issuance Costs and Discounts	$ 7,243	$ 6,854	$ 6,486
Debt Instrument, Interest Rate, Effective Percentage	4.90%	4.90%	4.90%
Debt Instrument, Face Amount	$ 172,500	$ 172,500		$ 172,500
Convertible senior notes \| 1.125% Convertible Senior Notes due 2018
Interest expense related to notes
Coupon interest	0	0	$ 377
Amortization of Debt Issuance Costs and Discounts	$ 0	$ 0	$ 2,756
Debt Instrument, Interest Rate, Effective Percentage			5.50%
Debt Instrument, Face Amount					$ 138,000