Other Income, net (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other income, net
Foreign exchange gain (loss)	$ (411,000)	$ 321,000	$ 165,000
Gain from local China government subsidy	3,800,000	808,000
Other income (expense)	(189,000)	(182,000)	187,000
Other income, net	$ 3,200,000	$ 947,000	$ 352,000