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Convertible Notes (Interest Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 17, 2017	Aug. 16, 2013
Interest expense related to notes
Amortization of Debt Issuance Costs and Discounts	$ 6,854	$ 9,243	$ 7,578
Additional paid in capital	$ 1,261,142	$ 1,226,588
Convertible senior notes | 1.375% convertible notes
Debt Instrument [Line Items]
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 [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		1.125%	1.125%		1.125%
Convertible notes
Interest expense related to notes
Total interest expense on convertible notes	$ 9,226	$ 11,991	$ 9,356
Additional paid in capital	111,300	111,300
Convertible notes | 1.375% convertible notes
Interest expense related to notes
Coupon interest	2,372	2,372	290
Amortization of Debt Issuance Costs and Discounts	$ 6,854	$ 6,486	$ 768
Debt Instrument, Interest Rate, Effective Percentage	4.90%	4.90%	4.90%
Debt Instrument, Face Amount	$ 172,500	$ 172,500		$ 172,500
Convertible notes | 1.125% Convertible Senior Notes due 2018
Interest expense related to notes
Coupon interest	0	377	$ 1,488
Amortization of Debt Issuance Costs and Discounts	$ 0	$ 2,756	$ 6,810
Debt Instrument, Interest Rate, Effective Percentage		5.50%	5.50%
Debt Instrument, Face Amount					$ 138,000

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