Convertible 4.5% Senior Notes (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016 USD ($) $ / shares
Convertible debt
Interest rate (as a percent)	4.50%		4.50%			4.50%
Convertible 4.5% Senior Notes outstanding	$ 2,100,000		$ 2,100,000
Interest expense	$ 23,000	$ 23,000	$ 47,000	$ 47,000
Convertible Notes
Convertible debt
Principal amount of debt								$ 100,000,000
Interest rate (as a percent)	4.50%		4.50%				4.50%
Interest expense			$ 4,700,000	$ 4,700,000
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