Convertible 4.5% Senior Notes (Details) - USD ($)		1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 01, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jan. 01, 2017	Dec. 31, 2016
Convertible debt
Convertible debt paid in cash	$ 1,100,000			$ 1,100,000
Interest rate (as a percent)				4.50%		4.50%
Interest expense			$ 24,000	$ 47,000	$ 71,000
Convertible 4.5% Senior Notes
Convertible debt
Principal amount of debt								$ 100,000,000.0
Convertible debt outstanding						$ 2,100,000
Shares issued with debt conversion (in shares)		238,777		238,777
Convertible debt paid in cash		$ 1,100,000
Debt converted		$ 1,000,000.0		$ 1,000,000.0	0
Interest rate (as a percent)				4.50%		4.50%	4.50%
Interest expense				$ 47,000	$ 71,000