Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax amount	$ 42,772	$ (9,907)	$ 9,835
Other comprehensive income (loss), tax (expense) benefit	(10,693)	2,477	(3,762)
Other comprehensive income (loss), before reclassifications, net of tax amount	31,420	(8,416)	(6,364)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	659	986	12,437
Other comprehensive income (loss), net of tax amount	32,079	(7,430)	6,073
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	44,136	(19,221)	(13,994)
Reclassification from accumulated other comprehensive income, current period, before tax amount			(15)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, before tax amount	3,605	3,761	4,721
Other comprehensive income (loss), before tax amount	47,741	(15,460)	(9,288)
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	(11,033)	4,805	5,353
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit			6
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(901)	(940)	(1,806)
Other comprehensive income (loss), tax (expense) benefit	(11,934)	3,865	3,553
Other comprehensive income (loss), before reclassifications, net of tax amount	33,103	(14,416)	(8,641)
Reclassification from accumulated other comprehensive income, current period, net of tax amount			(9)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	2,704	2,821	2,915
Other comprehensive income (loss), net of tax amount	35,807	(11,595)	(5,735)
Change in Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	(5,703)	3,742	(1,231)
Reclassification from accumulated other comprehensive income, current period, before tax amount	796	1,225	1,962
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	1,425	(936)	471
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(199)	(306)	(751)
Other comprehensive income (loss), before reclassifications, net of tax amount	(4,278)	2,806	(760)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	597	919	1,211
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	250	250	250
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(63)	(63)	(96)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	187	187	154
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	312	161
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(77)	(39)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	235	122
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	(4,345)	5,378	18,643
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	1,086	(1,344)	(7,131)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	(3,259)	4,034	11,512
Cash Flow Hedge Derivatives {Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	(145)	497	198
Reclassification from accumulated other comprehensive income, current period, before tax amount	(479)	(322)	282
Other comprehensive income (loss), before tax amount	(624)	175	480
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	36	(124)	(76)
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	119	80	(108)
Other comprehensive income (loss), tax (expense) benefit	155	(44)	(184)
Other comprehensive income (loss), before reclassifications, net of tax amount	(109)	373	122
Reclassification from accumulated other comprehensive income, current period, net of tax amount	(360)	(242)	174
Other comprehensive income (loss), net of tax amount	$ (469)	$ 131	296
Recognized Net Loss Due to Defined Benefit Plan Termination [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount			17,662
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit			(6,755)
Reclassification from accumulated other comprehensive income, current period, net of tax amount			$ 10,907