Other Comprehensive Income (Loss) - Components of Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AFS Debt Securities:
Unrealized holdings losses, pre-tax amount	$ 33,198	$ 26,743	$ (10,846)
Unrealized holdings losses, tax (expense) benefit	(7,138)	(5,750)	2,332
Unrealized holdings losses, after-tax amount	26,060	20,993	(8,514)
Less: reclassification adjustment for realized losses, pre-tax amount	0	(105)	(663)
Less: reclassification adjustment for realized losses, tax (expense) benefit	0	22	142
Less: reclassification adjustment for realized losses, after-tax amount	0	(83)	(521)
Net unrealized losses, pre-tax amount	33,198	26,848	(10,183)
Net unrealized losses, tax (expense) benefit	(7,138)	(5,772)	2,190
Net unrealized losses, after-tax amount	26,060	21,076	(7,993)
Cash Flow Hedges:
Net increase in fair value, pre-tax amount	1,214	(2,973)	1,066
Net increase in fair value, tax (expense) benefit	(260)	639	(229)
Net increase in fair value, after-tax amount	954	(2,334)	837
Less: effective portion reclassified into interest expense, pre-tax amount	(597)	(921)	(831)
Less: effective portion reclassified into interest expense, after-tax amount	(468)	(723)	(652)
Net increase in fair value, pre-tax amount	1,811	(2,052)	1,897
Net increase in fair value, tax (expense) benefit	(389)	441	(408)
Net increase in fair value, after-tax amount	1,422	(1,611)	1,489
Postretirement Plans:
Net actuarial loss, pre-tax amount	(1,282)	(1,918)	1,743
Net actuarial loss, tax (expense) benefit	277	412	(375)
Net actuarial loss, after-tax amount	(1,005)	(1,506)	1,368
Less: Amortization of net actuarial losses, pre-tax amount	(701)	(271)	(613)
Less: Amortization of net actuarial losses, tax (expense) benefit	151	59	131
Less: Amortization of net prior service credits, after-tax amount	(550)	(212)	(482)
Less: Amortization of net prior service credits, pre-tax amount	24	24	24
Less: Amortization of net prior service credits, tax (expense) benefit	(5)	(5)	(5)
Less: Amortization of net prior service credits, after-tax amount	19	19	19
Net loss on postretirement plans, pre-tax amount	(605)	(1,671)	2,332
Net loss on postretirement plans, tax (expense) benefit	131	358	(501)
Net (loss) gain on postretirement plans, net of tax	(474)	(1,313)	1,831
Other comprehensive income, pre-tax amount	34,404	23,125	(5,954)
Other comprehensive income, tax (expense) benefit	(7,396)	(4,973)	1,281
Other comprehensive income (loss)	27,008	18,152	(4,673)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: effective portion reclassified into interest expense, pre-tax amount	(597)	(921)	(831)
Less: Amounts reclassified from AOCI	(468)	(723)	(652)
Interest Expense, Borrowings and Subordinated Debentures
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	(1,603)	(707)	(831)
Less: effective portion reclassified into interest expense, tax (expense) benefit	345	151	179
Less: effective portion reclassified into interest expense, after-tax amount	(1,258)	(556)	(652)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: effective portion reclassified into interest expense, pre-tax amount	(1,603)	(707)	(831)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	(345)	(151)	(179)
Less: Amounts reclassified from AOCI	(1,258)	(556)	(652)
Interest Income, Interest and Fees on Loans
Cash Flow Hedges:
Less: effective portion reclassified into interest expense, pre-tax amount	1,006	(214)	0
Less: effective portion reclassified into interest expense, tax (expense) benefit	(216)	47	0
Less: effective portion reclassified into interest expense, after-tax amount	790	(167)	0
Accumulated Other Comprehensive Income (Loss) [Line Items]
Less: effective portion reclassified into interest expense, pre-tax amount	1,006	(214)	0
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	216	(47)	0
Less: Amounts reclassified from AOCI	$ 790	$ (167)	$ 0