Debt Convertible Notes (Details) - USD ($)		3 Months Ended	12 Months Ended
	Oct. 06, 2017	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2014	Dec. 31, 2017	Sep. 30, 2017	Dec. 08, 2014
Debt Instrument [Line Items]
Applicable base loan rate	1.50%				2.50%
Debt Instrument, Interest Rate, Stated Percentage, Initial Applicable Base Loan Rate	2.50%
Line of Credit Facility, Commitment Fee Percentage	0.10%
Line of credit facility, maximum borrowing capacity	$ 25,000,000.0
Less: current portion		$ (6,918,000)			$ (10,184,000)
Long-term debt		132,167,000			227,394,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.50%
Convertible Debt [Member] | Senior 3 Point 25 Percent Convertible Notes Due 2020 [Member]
Debt Instrument [Line Items]
Senior Credit Facility, applicable margin						3.25%
Principal amount of Convertible Senior Notes outstanding		97,095,000			167,314,000
Unamortized discount of liability component		(8,207,000)			(15,356,000)
Unamortized debt issuance costs				$ (4,200,000)
Net carrying amount of liability component		87,790,000			149,868,000
Less: current portion		(87,790,000)			0
Long-term debt		0			149,868,000
Carrying value of equity component, net of issuance costs		16,750,000			29,211,000		$ 30,700,000
Fair value of outstanding Convertible Senior Notes		$ 169,609,000			257,245,000
Remaining amortization period of discount on the liability component		2 years 9 months	3 years	6 years
Other Current Assets [Member] | Convertible Debt [Member] | Senior 3 Point 25 Percent Convertible Notes Due 2020 [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs		$ (1,098,000)			$ (2,090,000)