Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Income before income taxes	$ 61,811	$ 23,386	$ 37,026
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	1,474	(6,245)	(2,283)
Less: reclassification adjustment for net gains on sales realized in net income, Before Tax Amount	(48)	(518)	(1,265)
Less: reclassification adjustment for other-than-temporary impairment on equity securities realized in net income, Before Tax Amount	0	0	5
Total net unrealized gains (losses) on available-for-sale investment securities, Before Tax Amount	1,426	(6,763)	(3,543)
Cash flow hedge derivative, Before Tax Amount
Net change in fair value of interest rate swap, Before Tax Amount	49	(86)	(574)
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, Before Tax Amount	182	308	377
Total cash flow hedge derivative, Before Tax Amount	231	222	(197)
Defined benefit pension plans, Before Tax Amount
Net unrealized (losses) gains arising during the period, Before Tax Amount	(14)	(155)	(797)
Less: amortization of net actuarial loss included in net periodic pension costs, Before Tax Amount	1,227	1,321	1,362
Less: accretion of prior service cost included in net periodic pension costs, Before Tax Amount	(282)	(283)	(280)
Less: reclassification adjustment for net losses realized in net income	0	1,434	0
Total defined benefit pension plans, Before Tax Amount	931	2,317	285
Other comprehensive income (loss), Before Tax Amount	2,588	(4,224)	(3,455)
Total comprehensive income (loss), Before Tax Amount	64,399	19,162	33,571
Income taxes	17,717	3,881	9,758
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)	516	(2,186)	(799)
Less: reclassification adjustment for net gains on sales realized in net income, Tax Expense (Benefit)	(17)	(181)	(443)
Less: reclassification adjustment for other-than-temporary impairment on equity securities realized in net income, Tax Expense (Benefit)	0	0	2
Total net unrealized gains (losses) on available-for-sale investment securities, Tax Expense (Benefit)	499	(2,367)	(1,240)
Cash flow hedge derivative, Tax Expense (Benefit)
Net change in fair value of interest rate swap, Tax Expense (Benefit)	17	(30)	(201)
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, Tax Expense (Benefit)	64	108	132
Total cash flow hedge derivative, Tax Expense (Benefit)	81	78	(69)
Defined benefit pension plans, Tax Expense (Benefit)
Net unrealized (losses) gains arising during the period, Tax Expense (Benefit)	(5)	(54)	(279)
Less: amortization of net actuarial loss included in net periodic pension costs, Tax Expense (Benefit)	429	462	477
Less: accretion of prior service cost included in net periodic pension costs, Tax Expense (Benefit)	(99)	(99)	(98)
Less: reclassification adjustment for net losses realized in net income, Tax Expense (Benefit)	0	502	0
Total defined benefit pension plans, Tax Expense (Benefit)	325	811	100
Other comprehensive income (loss), Tax Expense (Benefit)	905	(1,478)	(1,209)
Total comprehensive income (loss), Tax Expense (Benefit)	18,622	2,403	8,549
Net income	44,094	19,505	27,268
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	958	(4,059)	(1,484)
Less: reclassification adjustment for net gains on sales realized in net income, Net of Tax Amount	(31)	(337)	(822)
Less: reclassification adjustment for other-than-temporary impairment on equity securities realized in net income, Net of Tax Amount	0	0	3
Total net unrealized gains (losses) on available-for-sale investment securities, Net of Tax Amount	927	(4,396)	(2,303)
Cash flow hedge derivative, Net of Tax Amount
Net change in fair value of interest rate swap, Net of Tax Amount	32	(56)	(373)
Less: reclassification adjustment for loss on termination of interest rate swap realized in net income, Net of Tax Amount	118	200	245
Total cash flow hedge derivative, Net of Tax Amount	150	144	(128)
Defined benefit pension plans, Net of Tax Amount
Net unrealized (losses) gains arising during the period, Net of Tax Amount	(9)	(101)	(518)
Less: amortization of net actuarial loss included in net periodic pension costs, Net of Tax Amount	798	859	885
Less: accretion of prior service cost included in net periodic pension costs, Net of Tax Amount	(183)	(184)	(182)
Less: Reclassification adjustment for net losses realized in net income, Net of Tax Amount	0	932	0
Total defined benefit pension plans, Net of Tax Amount	606	1,506	185
Other comprehensive income (loss), Net of Tax Amount	1,683	(2,746)	(2,246)
Total comprehensive income (loss), Net of Tax Amount	$ 45,777	$ 16,759	$ 25,022