Other (expense) income (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other (expense) income
Foreign exchange gain (loss)	$ 321,000	$ 165,000	$ (602,000)
Gain on available-for-sale securities			77,000
Gain from local China government subsidy	808,000
Other income (expense)	(182,000)	187,000	(28,000)
Other (expense) income, Total	$ 947,000	$ 352,000	$ (553,000)