Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax amount	$ (20,606)	$ 41,731
Other comprehensive income (loss), tax (expense) benefit	5,151	(10,433)
Other comprehensive income (loss), before reclassifications, net of tax amount	(15,817)	31,026
Reclassification from accumulated other comprehensive income, current period, net of tax amount	362	272
Other comprehensive income (loss), net of tax amount	(15,455)	31,298
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	(21,800)	40,507
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, before tax amount	711	861
Other comprehensive income (loss), before tax amount	(21,089)	41,368
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	5,450	(10,127)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(178)	(215)
Other comprehensive income (loss), tax (expense) benefit	5,272	(10,342)
Other comprehensive income (loss), before reclassifications, net of tax amount	(16,350)	30,380
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	533	646
Other comprehensive income (loss), net of tax amount	(15,817)	31,026
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
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(7)	(9)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	21	28
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	0
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	0	0
Reclassification from accumulated other comprehensive income, current period, net of tax amount	0	0
Change in Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	455	326
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(114)	(82)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	341	244
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	483	363
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(121)	(91)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	$ 362	$ 272