Unrecognized tax benefits (Details 8) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Interest (expense) recognized in income tax expense	$ (99,000)	$ (92,000)	$ 122,000
Accrued liability for payment of interest	$ 46,000	$ 54,000