Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Income before income taxes		$ 80,053	$ 60,686	$ 61,811
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		10,168	(7,280)	1,474
Less: reclassification adjustment for net gains on sales realized in net income, before tax amount	[1]	(54)	(10)	(48)
Total net unrealized gains (losses) on available-for-sale investment securities, before tax amount		10,114	(7,290)	1,426
Cash flow hedge derivative, before tax amount
Net change in fair value of interest rate swap, before tax amount		(420)	74	49
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, before tax amount	[2]	(22)	15	182
Total cash flow hedge derivative, before tax amount		(442)	89	231
Defined benefit pension plans, before tax amount
Net unrealized (losses) gains arising during the period, before tax amount		(2,308)	(1,603)	(14)
Less: amortization of net actuarial loss included in net periodic pension costs, before tax amount	[3]	1,176	1,124	1,227
Less: accretion of prior service cost included in net periodic pension costs, before tax amount	[3]	(181)	(283)	(282)
Total defined benefit pension plans, before tax amount		(1,313)	(762)	931
Other comprehensive income (loss), before tax amount		8,359	(7,963)	2,588
Total comprehensive income (loss), before tax amount		88,412	52,723	64,399
Income taxes		14,334	10,143	17,717
Net unrealized gains (losses) on available-for-sale investment securities, tax rxpense (benefit)
Net unrealized holding gains (losses) arising during the period, tax expense (benefit)		2,135	(1,529)	516
Less: reclassification adjustment for net gains on sales realized in net income, tax expense (benefit)	[1]	(11)	(2)	(17)
Total net unrealized gains (losses) on available-for-sale investment securities, tax expense (benefit)		2,124	(1,531)	499
Cash flow hedge derivative, tax expense (benefit)
Net change in fair value of interest rate swap, tax expense (benefit)		(88)	16	17
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, tax expense (benefit)	[2]	(5)	3	64
Total cash flow hedge derivative, tax expense (benefit)		(93)	19	81
Defined benefit pension plans, tax expense (benefit)
Net unrealized (losses) gains arising during the period, tax expense (benefit)		(485)	(337)	(5)
Less: amortization of net actuarial loss included in net periodic pension costs, tax expense (benefit)	[3]	248	236	429
Less: accretion of prior service cost included in net periodic pension costs, tax expense (benefit)	[3]	(38)	(59)	(99)
Total defined benefit pension plans, tax expense (benefit)		(275)	(160)	325
Other comprehensive income (loss), tax expense (benefit)		1,756	(1,672)	905
Total comprehensive income (loss), tax expense (benefit)		16,090	8,471	18,622
Net income		65,719	50,543	44,094
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		8,033	(5,751)	958
Less: reclassification adjustment for net gains on sales realized in net income, net of tax amount	[1]	(43)	(8)	(31)
Total net unrealized gains (losses) on available-for-sale investment securities, net of tax amount		7,990	(5,759)	927
Cash flow hedge derivative, net of tax amount
Net change in fair value of interest rate swap, net of tax amount		(332)	58	32
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, net of tax amount	[2]	(17)	12	118
Total cash flow hedge derivative, net of tax amount		(349)	70	150
Defined benefit pension plans, net of tax amount
Net unrealized (losses) gains arising during the period, net of tax amount		(1,823)	(1,266)	(9)
Less: amortization of net actuarial loss included in net periodic pension costs, net of tax amount	[3]	928	888	798
Less: accretion of prior service cost included in net periodic pension costs, net of tax amount	[3]	(143)	(224)	(183)
Total defined benefit pension plans, net of tax amount		(1,038)	(602)	606
Other comprehensive income (loss), net of tax amount		6,603	(6,291)	1,683
Total comprehensive income (loss), net of tax amount		$ 72,322	$ 44,252	$ 45,777

[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 pension cost (before tax amount). See Note 12, "Retirement Plans and Other Postretirement Benefits" for additional details.