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Convertible Notes (Interest Expense) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 17, 2017	Aug. 16, 2013
Interest expense related to notes
Amortization of Debt Issuance Costs and Discounts	$ 9,243	$ 7,578	$ 6,749
Additional paid in capital	$ 1,226,588	$ 1,212,798
Convertible Senior Notes [Member] \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	1.375%			1.375%
Convertible Senior Notes [Member] \| 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	$ 11,991	$ 9,356	8,302
Additional paid in capital	111,300	111,300
Convertible notes \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Interest expense related to notes
Coupon interest	2,372	290
Amortization of Debt Issuance Costs and Discounts	$ 6,486	768
Debt Instrument, Interest Rate, Effective Percentage	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	377	1,488	1,553
Amortization of Debt Issuance Costs and Discounts	$ 2,756	$ 6,810	$ 6,749
Debt Instrument, Interest Rate, Effective Percentage	5.50%	5.50%
Debt Instrument, Face Amount	$ 0	$ 81,207			$ 138,000