Convertible 3.75% Senior Notes (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Debt issuance costs	$ 2,300,000	$ 3,200,000
Convertible Notes Payable
Debt Instrument [Line Items]
Face amount of the Notes	143,750,000	143,750,000
Unamortized discount	20,517,000	29,160,000
Debt issuance costs	2,256,000	3,199,000
Carrying amount of the Notes	$ 120,977,000	$ 111,391,000