Leases - Undiscounted Cash Payment Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Undiscounted cash payment obligations for its operating lease liabilities
2020	$ 3,065
2021	2,183
Total undiscounted operating lease payments	5,248
Imputed interest expenses	(564)
Total liabilities	4,684
Current portion of lease liabilities	2,608
Operating lease liability, net of current portion	2,076
Rent expense under operating leases	$ 2,592	$ 1,800	$ 1,700