Income taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Income Tax Examination [Line Items]
Income tax benefit
Operating loss carryforwards	21,400
Increase in valuation allowance	712	$ 998
Federal [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards	21,400
Federal [Member] | Winthrop [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards	$ 1,300
Federal [Member] | Minimum [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards, expiration date	Jan. 01, 2031
Federal [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2036
New York State [Member] | Winthrop [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards	$ 8,500
New York State [Member] | Pre-apportioned [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards	19,200
New York State [Member] | Post-apportioned [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards	$ 1,400
New York State [Member] | Minimum [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards, expiration date	Jan. 01, 2017
New York State [Member] | Maximum [Member]
Income Tax Examination [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2036