Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Income before income taxes		$ 56,897	$ 80,053	$ 60,686
Net unrealized gains (losses) on available-for-sale investment securities, before tax amount
Net unrealized holding gains (losses) arising during the period, before tax amount		2,346	10,168	(7,280)
Provision for credit losses, before tax		569	0	0
Less: reclassification adjustment for net gains on sales realized in net income, before tax amount	[1]	(871)	(54)	(10)
Total net unrealized gains (losses) on available-for-sale investment securities, before tax amount		2,044	10,114	(7,290)
Cash flow hedge derivative, before tax amount
Net change in fair value of interest rate swap, before tax amount		(553)	(420)	74
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, before tax amount	[2]	254	(22)	15
Total cash flow hedge derivative, before tax amount		(299)	(442)	89
Defined benefit pension plans, before tax amount
Net unrealized (losses) gains arising during the period, before tax amount		(3,772)	(2,308)	(1,603)
Less: amortization of net actuarial loss included in net periodic pension costs, before tax amount	[3]	1,202	1,176	1,124
Less: accretion of prior service cost included in net periodic pension costs, before tax amount	[3]	0	(181)	(283)
Total defined benefit pension plans, before tax amount		(2,570)	(1,313)	(762)
Other comprehensive income (loss), before tax amount		(825)	8,359	(7,963)
Total comprehensive income (loss), before tax amount		56,072	88,412	52,723
Income tax expense		9,981	14,334	10,143
Net unrealized gains (losses) on available-for-sale investment securities, tax expense (benefit)
Net unrealized holding gains (losses) arising during the period, tax expense (benefit)		493	2,135	(1,529)
Provision for credit losses, tax expense (benefit)		119	0	0
Less: reclassification adjustment for net gains on sales realized in net income, tax expense (benefit)	[1]	(183)	(11)	(2)
Total net unrealized gains (losses) on available-for-sale investment securities, tax expense (benefit)		429	2,124	(1,531)
Cash flow hedge derivative, tax expense (benefit)
Net change in fair value of interest rate swap, tax expense (benefit)		(116)	(88)	16
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, tax expense (benefit)	[2]	53	(5)	3
Total cash flow hedge derivative, tax expense (benefit)		(63)	(93)	19
Defined benefit pension plans, tax expense (benefit)
Net unrealized (losses) gains arising during the period, tax expense (benefit)		(792)	(485)	(337)
Less: amortization of net actuarial loss included in net periodic pension costs, tax expense (benefit)	[3]	252	248	236
Less: accretion of prior service cost included in net periodic pension costs, tax expense (benefit)	[3]	0	(38)	(59)
Total defined benefit pension plans, tax expense (benefit)		(540)	(275)	(160)
Other comprehensive income (loss), tax expense (benefit)		(174)	1,756	(1,672)
Total comprehensive income (loss), tax expense (benefit)		9,807	16,090	8,471
Net income		46,916	65,719	50,543
Net unrealized gains (losses) on available-for-sale investment securities, net of tax amount
Net unrealized holding gains (losses) arising during the period, net of tax amount		1,853	8,033	(5,751)
Provision for credit losses, net of tax		450	0	0
Less: reclassification adjustment for net gains on sales realized in net income, net of tax amount	[1]	(688)	(43)	(8)
Total net unrealized gains (losses) on available-for-sale investment securities, net of tax amount		1,615	7,990	(5,759)
Cash flow hedge derivative, net of tax amount
Net change in fair value of interest rate swap, net of tax amount		(437)	(332)	58
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, net of tax amount	[2]	201	(17)	12
Total cash flow hedge derivative, net of tax amount		(236)	(349)	70
Defined benefit pension plans, net of tax amount
Net unrealized (losses) gains arising during the period, net of tax amount		(2,980)	(1,823)	(1,266)
Less: amortization of net actuarial loss included in net periodic pension costs, net of tax amount	[3]	950	928	888
Less: accretion of prior service cost included in net periodic pension costs, net of tax amount	[3]	0	(143)	(224)
Total defined benefit pension plans, net of tax amount		(2,030)	(1,038)	(602)
Other comprehensive income (loss), net of tax amount		(651)	6,603	(6,291)
Total comprehensive income (loss), net of tax amount		$ 46,265	$ 72,322	$ 44,252

[1] (1) Included in net gain on sales of investment securities on the consolidated statements of income (before tax amount).

[2] (2) Included in interest expense on demand deposits on the consolidated statements of income (before tax amount).

[3] (3) These accumulated other comprehensive loss components are included in the computation of net periodic benefit cost (before tax amount). See Note 12, "Retirement Plans and Other Postretirement Benefits" for additional details.