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Convertible Notes (Schedule of Notes) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Nov. 17, 2017	Aug. 16, 2013
Debt Instrument [Line Items]
Total convertible notes	$ 141,934	$ 213,898
Less current portion	0	78,451
Total long-term convertible notes	141,934	135,447
Convertible notes
Debt Instrument [Line Items]
Total convertible notes	172,500	253,707
Convertible notes \| Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Debt Instrument [Line Items]
Face Value	172,500	172,500	$ 172,500
Unamortized discount	(28,517)	(34,506)
Unamortized Debt Issuance Expense	(2,049)	(2,547)
Convertible notes \| 1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Face Value	0	81,207		$ 138,000
Unamortized discount	0	(2,547)
Unamortized Debt Issuance Expense	$ 0	$ (209)