Convertible 4.5% Senior Notes (Details)	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($) shares	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares
Convertible debt
Interest expense	$ 23,000	$ 1,125,000	$ 47,000		$ 2,250,000
Convertible Notes
Convertible debt
Interest rate (as a percent)	4.50%		4.50%	4.50%
Principal amount of debt				$ 2,100,000		$ 100,000,000
Proceeds from issuance of debt						96,600,000
Issuance of debt transaction costs						3,400,000
Debt converted				$ 97,900,000
Shares issued with debt conversion (in shares) | shares				26,160,187
Shares issued with debt conversion adjustment (in shares) | shares				2,784,870
Principal amount of debt for conversion calculations						$ 1,000
Ratio issued upon conversion						238.7775
Initial conversion price (in dollars per share) | $ / shares						$ 4.19