Other Long-Term Obligations Convertible Senior Notes (Details) - Convertible Subordinated Debt - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Convertible Senior Notes at 5.00% February 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 150,000,000	$ 150,000,000
Interest rate (as a percent)	5.00%
Convertible 2021 debt conversion liability	$ 55,527,000	53,154,000
Convertible Senior Notes at 4.50% February 2022 [Domain]
Debt Instrument [Line Items]
Convertible Debt 2022 Conversion Feature, Fair Value	55,224,000	53,414,000
Debt Instrument, Face Amount	$ 120,000,000	$ 120,000,000
Interest rate (as a percent)	4.50%