Convertible 4.5% Senior Notes (Details)	1 Months Ended	3 Months Ended										6 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) $ / shares	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($) $ / shares
Convertible debt
Interest rate (as a percent)		4.50%												4.50%
Proceeds from issuance of debt																	$ 96,608,000
Issuance of debt transaction costs																	3,392,000
Interest expense		$ 25,000	$ 23,000	$ 23,000	$ 24,000	$ 28,000	$ 762,000	$ 1,125,000	$ 1,125,000	$ 1,099,000	$ 1,150,000		$ 2,249,000	$ 95,000	$ 3,040,000	$ 2,387,000	138,000
Convertible Notes
Convertible debt
Interest rate (as a percent)		4.50%				4.50%				4.50%		4.50%	4.50%	4.50%	4.50%	4.50%
Principal amount of debt		$ 2,100,000								$ 100,000,000			$ 100,000,000	$ 2,100,000		$ 100,000,000
Proceeds from issuance of debt																$ 96,600,000
Debt converted												$ 97,900,000			$ 96,900,000
Shares issued with debt conversion (in shares) | shares												26,160,187
Shares issued with debt conversion adjustment (in shares) | shares												2,784,870
Debt Conversion other amount															1,000,000
Debt conversion loss															22,900,000
Accrued interest forfeited															743,000
Charges to paid in capital															2,500,000
Transaction costs														1,700,000
Interest expense													$ 2,200,000	95,000	3,000,000		138,000
Principal amount of debt for conversion calculations	$ 1,000					$ 1,000						$ 1,000			$ 1,000		$ 1,000
Ratio issued upon conversion	238.7775
Initial conversion price (in dollars per share) | $ / shares	$ 4.19																$ 4.19
Deferred financing costs		$ 50,000												$ 50,000