Accumulated Other Comprehensive (Loss) Income, Reclassifications out of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	$ 10,210	$ 9,003
Income tax expense (benefit)	6,561	6,106
Net income	10,947	10,409
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items]
Net income	(24)	33
Amortization of Pension and Postretirement Benefit Items [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense (benefit)	16	(23)
Net income	(24)	33
Amortization of Net Actuarial (Gain) Loss [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	(63)	33
Amortization of Prior Service Cost [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits	$ 23	$ 23