Summary of significant accounting policies - Foreign Currency Translation (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax	$ 44,000	$ (84,000)	$ 77,000
Foreign Currency Transaction Gain (Loss), before Tax		$ 225,000	$ 169,000