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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before tax amount	$ 5,277	$ (8,274)	$ 46,428	$ (42,262)
Other comprehensive income (loss), tax (expense) benefit	(1,319)	2,067	(11,607)	10,565
Other comprehensive income (loss), before reclassifications, net of tax amount			34,323	(32,591)
Reclassification from accumulated other comprehensive income, current period, net of tax amount			498	894
Other comprehensive income (loss), net of tax amount	3,958	(6,207)	34,821	(31,697)
Securities Available for Sale and Transferred Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	4,154	(9,660)	43,057	(46,974)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, before tax amount	849	973	2,852	2,909
Other comprehensive income (loss), before tax amount	5,003	(8,687)	45,909	(44,065)
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	(1,038)	2,414	(10,764)	11,743
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(212)	(243)	(713)	(727)
Other comprehensive income (loss), tax (expense) benefit	(1,250)	2,171	(11,477)	11,016
Other comprehensive income (loss), before reclassifications, net of tax amount	3,116	(7,246)	32,293	(35,231)
Other comprehensive income (loss), change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	637	730	2,139	2,182
Other comprehensive income (loss), net of tax amount	3,753	(6,516)	34,432	(33,049)
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	62	63	187	188
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(16)	(16)	(47)	(48)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	46	47	140	140
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	47	41	141	121
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(13)	(10)	(36)	(29)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	34	31	105	92
Change in Net Actuarial Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification from accumulated other comprehensive income, current period, before tax amount	247	361	749	1,092
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(61)	(90)	(187)	(273)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	186	271	562	819
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	356	465	1,077	1,401
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	(90)	(116)	(270)	(350)
Reclassification from accumulated other comprehensive income, current period, net of tax amount	266	349	807	1,051
Cash Flow Hedge Derivatives {Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), before reclassifications, before tax amount	19	52	(145)	611
Reclassification from accumulated other comprehensive income, current period, before tax amount	(101)	(104)	(413)	(209)
Other comprehensive income (loss), before tax amount	(82)	(52)	(558)	402
Other comprehensive income (loss), before reclassifications, tax (expense) benefit	(5)	(13)	36	(153)
Reclassification from accumulated other comprehensive income, current period, tax (expense) benefit	26	25	104	52
Other comprehensive income (loss), tax (expense) benefit	21	12	140	(101)
Other comprehensive income (loss), before reclassifications, net of tax amount	14	39	(109)	458
Reclassification from accumulated other comprehensive income, current period, net of tax amount	(75)	(79)	(309)	(157)
Other comprehensive income (loss), net of tax amount	$ (61)	$ (40)	$ (418)	$ 301