Unrecognized tax benefits (Details 7) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 0	$ 105,000	$ 7,845,000
Settlements	0	0	(7,740,000)
Lapse of statute of limitations	0	(105,000)	0
Balance at end of year	0	0	105,000
Interest (income) expense recognized in income tax expense	(92,000)	122,000	$ 387,000
Amount of interest income accrued for an overpayment of income taxes.	$ 54,000
Accrued liability for payment of interest		$ 94,000