Convertible 4.5% Senior Notes (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	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%
Convertible 4.5% Senior Notes outstanding	$ 2,100,000
Interest expense	$ 24,000	$ 24,000
Convertible Notes
Convertible debt
Principal amount of debt						$ 100,000,000
Interest rate (as a percent)	4.50%				4.50%
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