Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax amount	$ 7,819	$ 3,822	$ (12,787)	$ 45,554
Other comprehensive income (loss), tax (expense) benefit	(1,954)	(955)	3,197	(11,389)
Other comprehensive income (loss), before reclassifications, net of tax amount			(10,306)	33,595
Reclassification from accumulated other comprehensive income, current period, net of tax amount			716	570
Other comprehensive income (loss), net of tax amount	5,865	2,867	(9,590)	34,165
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	6,612	2,648	(15,188)	43,155
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, before tax amount	736	778	1,447	1,639
Other comprehensive income (loss), before tax amount	7,348	3,426	(13,741)	44,794
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	(1,653)	(662)	3,797	(10,789)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(184)	(195)	(362)	(410)
Other comprehensive income (loss), tax (expense) benefit	(1,837)	(857)	3,435	(11,199)
Other comprehensive income (loss), before reclassifications, net of tax amount	4,959	1,986	(11,391)	32,366
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	552	583	1,085	1,229
Other comprehensive income (loss), net of tax amount	5,511	2,569	(10,306)	33,595
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	38	56	75
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(7)	(10)	(14)	(19)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	21	28	42	56
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	0	40	0	40
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	0	(10)	0	(10)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	0	30	0	30
Change in Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	443	318	898	645
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(110)	(78)	(224)	(161)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	333	240	674	484
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	471	396	954	760
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(117)	(98)	(238)	(190)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	$ 354	$ 298	$ 716	$ 570