Income Taxes (Details) $ in Thousands, £ in Millions	3 Months Ended					12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)
Income Taxes.
Benefit from income taxes	$ 138	$ 331	$ (135)	$ (80)	$ 396	$ 254	$ 396	$ 23
Increases recorded to income tax provision								2,500
U.K. operating loss carryforwards										$ 41,700
Federal and state net operating loss carryforwards										0
Amount subject to offset each year | £									£ 5.0
Incremental percentage of U.K. taxable profits						50.00%
Research credit carryforwards							197			434
Uncertain tax positions							0			0
Interest and penalties accrued							$ 0	$ 0		0
Federal
Income Taxes.
Research credit carryforwards										300
State
Income Taxes.
Research credit carryforwards										$ 200