Income Taxes - Unrecognized Tax Benefits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Unrecognized tax benefits at the beginning of year	$ 14,620,000	$ 14,553,000	$ 2,223,000
Gross increases - current year tax positions			13,899,000
Gross increases - prior year tax positions	360,000	67,000
Gross decreases - prior year tax positions			(1,569,000)
Unrecognized tax benefits at end of year	14,980,000	14,620,000	$ 14,553,000
Interest and penalties in year-end balance	427,000	67,000
Interest expense related to uncertain tax positions	$ 360,000	$ 67,000