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Convertible Notes (Schedule of Notes) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Nov. 17, 2017	Dec. 31, 2016	Aug. 16, 2013
Debt Instrument [Line Items]
Total convertible notes	$ 213,898		$ 126,167
Less current portion	78,451		0
Total long-term convertible notes	135,447		126,167
Convertible notes
Debt Instrument [Line Items]
Total convertible notes	253,707		138,000
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	0
Unamortized discount	(34,506)		0
Unamortized Debt Issuance Expense	(2,547)		0
Convertible notes \| 1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Face Value	81,207		138,000	$ 138,000
Unamortized discount	(2,547)		(10,913)
Unamortized Debt Issuance Expense	$ (209)		$ (920)