Accumulated Other Comprehensive Income (Loss), Reclassifications out of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Net gain on securities transactions	[1]	$ 0		$ 0		$ 1,155	$ 0
Salaries and employee benefits		11,648		11,711		23,021	23,162
Income tax effect		3,921		4,877		8,227	9,521
Net income		11,254	$ 13,313	14,667	$ 14,558	24,567	29,225
Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Net income		141		62		1,155	98
Unrealized Gains on Securities Available for Sale [Member] | Reclassification Out of Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment Out of Accumulated Other Comprehensive Income [Abstract]
Net gain on securities transactions		0		0		1,155	0
Income tax effect		0		0		(300)	0
Net income		0		0		855	0
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 effect		(51)		(22)		(107)	(35)
Net income		141		62		300	98
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		143		106		309	178
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		$ 49		$ (22)		$ 98	$ (45)

[1] Not within the scope of ASC 606.