Income Taxes - Unrecognized tax benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Unrecognized tax benefits	$ 119	$ 81
Unrecognized tax benefits
Beginning balance	119	81
Gross increase - tax positions in prior periods		38
Lapse in statutes of limitations	(30)
Ending balance	89	119
Accounts payable and accrued expenses
Unrecognized tax benefits	100	100
Unrecognized tax benefits
Beginning balance	100
Ending balance	$ 100	$ 100