Convertible 4.5% Senior Notes (Details)	3 Months Ended												6 Months Ended	12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	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, 2017 USD ($) shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) $ / shares
Convertible debt
Interest rate (as a percent)	4.50%				4.50%									4.50%	4.50%
Interest expense	$ 24,000	$ 24,000	$ 23,000	$ 24,000	$ 25,000	$ 23,000	$ 23,000	$ 24,000	$ 28,000	$ 762,000	$ 1,125,000	$ 1,125,000		$ 95,000	$ 95,000	$ 3,040,000
Convertible Notes
Convertible debt
Interest rate (as a percent)	4.50%								4.50%				4.50%	4.50%		4.50%	4.50%
Principal amount of debt									$ 2,100,000				$ 2,100,000			$ 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			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		2,500,000
Transaction costs														1,700,000
Interest expense														95,000	$ 95,000	$ 3,000,000.0
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
Transaction costs of convertible note	3,400,000													3,400,000
Deferred financing costs	$ 50,000													$ 50,000