Long-Term Debt Components of Convertible Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2019	Dec. 31, 2018	Jun. 30, 2017	Mar. 31, 2016
Debt Instrument [Line Items]
Issuance of warrants		$ 12,376,000
Convertible 2021 note hedge asset			$ 0	$ 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	$ 120,000,000
Issuance of warrants	$ 14,100,000
Interest rate (as a percent)			4.50%
Unamortized discount			$ (17,438,000)	(21,476,000)
Long-term debt			100,181,000	95,473,000
Debt Instrument, Fee Amount			(2,381,000)	(3,051,000)
Debt Instrument, Net Carrying Amount			100,181,000	95,473,000
Convertible Subordinated Debt | Convertible Senior Notes at 5.00% February 2021 [Member]
Debt Instrument [Line Items]
Convertible 2021 debt conversion liability			0	1,458,000
Debt Instrument, Face Amount			$ 134,000,000	150,000,000		$ 150,000,000
Interest rate (as a percent)			5.00%
Unamortized discount			$ (10,356,000)	(17,193,000)
Long-term debt			122,100,000	130,260,000
Debt Instrument, Fee Amount			(1,544,000)	(2,547,000)
Debt Instrument, Net Carrying Amount			$ 122,100,000	$ 130,260,000