Convertible 4.5% Senior Notes (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) item $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) $ / shares
Convertible debt
Interest rate (as a percent)	4.50%	4.50%	4.50%	4.50%	4.50%
Proceeds from issuance of debt					$ 96,608,000
Issuance of debt transaction costs					3,392,000
Interest expense		$ 1,125,000	$ 138,000	$ 2,250,000	$ 138,000
Convertible Notes
Convertible debt
Interest rate (as a percent)	4.50%	4.50%	4.50%	4.50%	4.50%
Principal amount of debt	$ 100,000,000		$ 100,000,000		$ 100,000,000
Proceeds from issuance of debt	96,600,000
Issuance of debt transaction costs	3,400,000
Interest expense				$ 2,300,000	138,000
Principal amount of debt for conversion calculations	$ 1,000		$ 1,000		$ 1,000
Ratio issued upon conversion	238.7775
Initial conversion price (in dollars per share) | $ / shares	$ 4.19		$ 4.19		$ 4.19
Principal amount of notes to be repurchased under certain conditions (as a percent)	100.00%
Principal amount of notes to be repurchased upon default (as a percent)	100.00%
Additional interest for reporting requirement failure (as a percent)	0.50%
Default cure period	90 days
Additional interest for restricted event (as a percent)	0.50%
Number of additional interest rate percentage penalty circumstances | item	2
Maximum additional interest rate (as a percent)	0.50%