Commitments and Contingencies - Components of lease expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating lease costs:
Operating lease rental expense	$ 1,187
Sublease income	(1,263)
Total operating lease costs	(76)
Finance lease costs:
Amortization of right-of-use assets	11
Interest on lease liabilities	2
Total finance lease costs	$ 13
Operating lease costs:
Operating lease rental expense		$ 1,532	$ 1,355
Sublease income		(139)
Total operating lease costs		1,393	1,355
Finance lease costs:
Amortization of right-of-use assets		12	8
Interest on lease liabilities		3	2
Total finance lease costs		$ 15	$ 10