Income taxes - Schedule of Reconciliation of Effective Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
U.S. federal statutory income tax rate	$ 2,348	$ 9,008	$ 15,368
U.S. state and local income taxes, net of U.S. federal income tax benefits	373	2,033	2,131
Unrecognized tax benefit	589	6	1,244
Tax exempt income, net of interest expense	(696)	(528)	(715)
Non-deductible regulatory settlements	0	5,298	0
Business promotion and other non-deductible expenses	577	655	660
Insurance proceeds, non-taxable	0	(65)	(597)
Adjustment to reflect prior year tax return filings	397	256	(251)
Tax rate change on deferred income taxes	116	241	208
Tax rate differential on foreign operations	145	(447)	185
Other	(36)	(282)	(477)
Total	$ 3,813	$ 16,175	$ 17,756
U.S. federal statutory income tax rate, percent	35.00%	35.00%	35.00%
U.S. state and local income taxes, net of U.S. federal income tax benefits, percent	5.50%	7.90%	4.90%
Unrecognized tax benefits, percent	8.80%	0.00%	2.80%
Tax exempt income, net of interest expense, percent	(10.40%)	(2.00%)	(1.60%)
Non-deductible regulatory settlements, percent	0.00%	20.60%	0.00%
Business promotion and other non-deductible expenses, percent	8.60%	2.50%	1.50%
Insurance proceeds, non-taxable, percent	0.00%	(0.30%)	(1.40%)
Adjustment to reflect prior year tax return filings, percent	5.90%	1.00%	(0.60%)
Tax rate change on deferred income taxes, percent	1.70%	0.90%	0.50%
Non-U.S. operations, percent	2.20%	(1.70%)	0.40%
Other, percent	(0.50%)	(1.10%)	(1.10%)
Total income tax expense, percent	56.80%	62.80%	40.40%