Income taxes (Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract
Income Tax Reconciliation State And Local Income Taxes	$ 26,555	$ 33,220	$ (7,823)
Income Tax Reconciliation Tax Exempt Income	(9,506)	(11,178)	(8,625)
Income Tax Reconciliation Disallowed net operating loss carryover	281	1,406	556
Income Tax Reconciliation Change In Deferred Tax Assets Valuation Allowance	(305)	(9)	(2,219)
Interest and Penalties Released Related to Unrecognized Tax Benefits	(775)	(135)	(385)
Income Tax Loan Basis Change Effect	0	0	0
Income Tax Reconciliation Capital (gain) loss at preferential rate	(279)	2,394	283
Income Tax Reconciliation Other Adjustments	(528)	296	659
Total Income Tax Expense (Benefit)	$ 15,443	$ 25,994	$ (17,554)
EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract
Statutary tax rate	39.00%	39.00%	(39.00%)
Tax effect of exempt income, net	(13.96%)	(13.12%)	(43.00%)
Effective Income Tax Rate Disallowed Net Operating Loss Carryover	(0.41%)	(1.65%)	(2.77%)
Effective Income Tax Rate Reconciliation Change In Deferred Tax Assets Valuation Allowance	(0.45%)	(0.01%)	(11.06%)
Effective Income Tax Rate Reconciliation Tax Contingencies	(1.14%)	(0.16%)	(1.92%)
Effective Income Tax Rate Loan Basis Change Effect	0.00%	0.00%	0.00%
Effective Income Tax Rate Capital Loss (Gain) at Preferential Rate	(0.41%)	2.81%	1.41%
Other items, net	(0.79%)	0.34%	3.28%
Total Income Tax Expense (Benefit)	22.66%	30.51%	(87.52%)