Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes.
Benefit from income taxes	$ 55	$ 465	$ 97	$ 107	$ 138	$ 331	$ (135)	$ (80)	$ 724	$ 254	$ 396
Increases recorded to income tax provision									8,000
U.K. operating loss carryforwards	72,000								72,000
Federal and state net operating loss carryforwards	0								0
Research credit carryforwards	1,233				434				1,233	434
Uncertain tax positions	0				0				0	0	0
Interest and penalties accrued	0				$ 0				0	$ 0	$ 0
Federal
Income Taxes.
Research credit carryforwards	800								800
State
Income Taxes.
Research credit carryforwards	$ 400								$ 400