Inventories - Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for excess and obsolete inventories:
Balance, beginning of period	$ 1,210	$ 1,118	$ 1,382
Provisions for excess and obsolete inventories	1,454	152	102
Inventories written off	(20)	(60)	(366)
Balance, end of period	$ 2,644	$ 1,210	$ 1,118