Accumulated Other Comprehensive Income (Loss), Reclassifications out of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Salaries and employee benefits	$ 11,725			$ 10,761			$ 34,887	$ 31,924
Income tax benefit	4,790			4,935			14,311	14,470
Net income	14,708	$ 14,667	$ 14,558	15,199	$ 15,405	$ 14,808	43,933	45,412
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Net income	87			194			261	292
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	(31)			(68)			(92)	(103)
Net income	87			194			261	292
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	35			189			103	367
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	$ 83			$ 73			$ 250	$ 28