Accumulated Other Comprehensive Income (Loss), Reclassifications out of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Salaries and employee benefits	$ 11,451	$ 10,422
Income tax benefit	4,644	4,731
Net income	14,558	14,808
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Net income	98	(36)
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 [Abstract]
Income tax benefit	(35)	13
Net income	98	(36)
Amortization of Net Actuarial Gain (Loss) [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Salaries and employee benefits	48	(72)
Amortization of Prior Service Cost [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Salaries and employee benefits	$ 85	$ 23