Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax amount	$ (11,252)	$ (6,507)	$ (24,039)	$ 39,047
Other comprehensive income (loss), tax (expense) benefit	2,813	(1,626)	6,010	(9,763)
Other comprehensive income (loss), before reclassifications, net of tax amount			(19,236)	28,417
Reclassification from accumulated other comprehensive income, current period, net of tax amount			1,207	867
Other comprehensive income (loss), net of tax amount	(8,439)	(4,881)	(18,029)	29,284
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	(12,543)	(7,707)	(27,731)	35,448
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, before tax amount	636	804	2,083	2,443
Other comprehensive income (loss), before tax amount	(11,907)	(6,903)	(25,648)	37,891
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	3,136	1,926	6,933	(8,863)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(159)	(201)	(521)	(611)
Other comprehensive income (loss), tax (expense) benefit	2,977	1,725	6,412	(9,474)
Other comprehensive income (loss), before reclassifications, net of tax amount	(9,407)	(5,781)	(20,798)	26,585
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	477	603	1,562	1,832
Other comprehensive income (loss), net of tax amount	(8,930)	(5,178)	(19,236)	28,417
Net Change in Prior Service Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	28	37	84	112
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(7)	(9)	(21)	(28)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	21	28	63	84
Recognized Net Loss Due to Lump Sum Settlements [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	183	40	183	80
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(46)	(10)	(46)	(20)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	137	30	137	60
Change in Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	444	319	1,342	964
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(111)	(80)	(335)	(241)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	333	239	1,007	723
Pension and Other Postretirement Benefit Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	655	396	1,609	1,156
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(164)	(99)	(402)	(289)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	$ 491	$ 297	$ 1,207	$ 867