Long-Term Debt Components of Convertible Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Issuance of warrants		$ 12,376,000
Convertible 2022 note hedge asset			$ 7,532,000	$ 1,028,000
Convertible Subordinated Debt | Convertible Senior Notes at 4.50% February 2022 [Domain]
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 120,000,000	120,000,000
Issuance of warrants	$ 14,100,000
Interest rate (as a percent)			4.50%
Unamortized discount			$ (20,161,000)	(21,476,000)
Long-term debt			97,011,000	95,473,000
Debt Instrument, Fee Amount			(2,828,000)	(3,051,000)
Debt Instrument, Net Carrying Amount			97,011,000	95,473,000
Convertible Subordinated Debt | Convertible Senior Notes at 5.00% February 2021 [Member]
Debt Instrument [Line Items]
Convertible 2021 debt conversion liability			8,173,000	1,458,000
Debt Instrument, Face Amount			$ 150,000,000	150,000,000
Interest rate (as a percent)			5.00%
Unamortized discount			$ (15,408,000)	(17,193,000)
Long-term debt			132,345,000	130,260,000
Debt Instrument, Fee Amount			(2,247,000)	(2,547,000)
Debt Instrument, Net Carrying Amount			$ 132,345,000	$ 130,260,000