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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, Before tax amount	$ (9,275)	$ 4,585	$ (37,314)	$ 6,870
Reclassification adjustment for net (gains) losses realized in net income, Before tax amount	0	(1)	0	(1)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	971	1,189	1,936	2,421
Total securities available for sale and transferred securities, Before tax amount	(8,304)	5,773	(35,378)	9,290
Pension and other postretirement benefit plans:
Net change in prior service costs, Before tax amount	62	62	125	125
Recognized net loss due to lump sum settlements, Before tax amount	40	0	80	0
Change in net actuarial loss, Before tax amount	363	780	731	1,567
Recognized net loss due to defined benefit plan termination, Before tax amount	0	16,991	0	16,991
Total pension and other postretirement benefit plans, Before tax amount	465	17,833	936	18,683
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, Before tax amount	132	(209)	559	(152)
Reclassification adjustment for (gain) loss realized in net income, Before tax amount	(99)	78	(105)	177
Total cash flow hedge derivatives, Before tax amount	33	(131)	454	25
Total other comprehensive income (loss), Before tax amount	(7,806)	23,475	(33,988)	27,998
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, tax (expense) benefit	2,320	(1,753)	9,329	(2,627)
Reclassification adjustment for net (gains) losses realized in net income, tax (expense) benefit	0	0	0	0
Change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(243)	(455)	(484)	(926)
Total securities available for sale and transferred securities, tax (expense) benefit	2,077	(2,208)	8,845	(3,553)
Reclassification adjustment for net (gains) losses realized in net income, tax (expense) benefit	0	0	0	0
Pension and other postretirement benefit plans:
Net change in prior service costs, tax (expense) benefit	(16)	(24)	(32)	(48)
Recognized net loss due to lump sum settlements, tax (expense) benefit	(10)	0	(19)	0
Change in net actuarial loss, tax (expense) benefit	(91)	(298)	(183)	(599)
Recognized net loss due to defined benefit plan termination, tax (expense) benefit	0	(6,499)	0	(6,499)
Total pension and other postretirement benefit plans, tax (expense) benefit	(117)	(6,821)	(234)	(7,146)
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, tax (expense) benefit	(33)	80	(140)	58
Reclassification adjustment for (gain) loss realized in net income, tax (expense) benefit	26	(30)	27	(68)
Total cash flow hedge derivatives, tax (expense) benefit	(7)	50	(113)	(10)
Total other comprehensive income (loss), tax (expense) benefit	1,953	(8,979)	8,498	(10,709)
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period	(6,955)	2,832	(27,985)	4,243
Reclassification adjustment for net (gains) losses realized in net income, net of tax	0	(1)	0	(1)
Change in net unrealized holding loss on securities transferred to held to maturity	728	734	1,452	1,495
Total securities available for sale and transferred securities, net of tax	(6,227)	3,565	(26,533)	5,737
Pension and other postretirement benefit plans:
Net change in prior service costs, net of tax amount	46	38	93	77
Recognized net loss due to lump sum settlements, net of tax amount	30	0	61	0
Change in net actuarial loss, net of tax amount	272	482	548	968
Recognized net loss due to defined benefit plan termination, net of tax	0	10,492	0	10,492
Total pension and other postretirement benefit plans, net of tax amount	348	11,012	702	11,537
Cash flow hedge derivatives:
Change in the accumulated gain (loss) on effective cash flow hedge derivatives	99	(129)	419	(94)
Reclassification adjustment for (gain) loss realized in net income, net of tax	(73)	48	(78)	109
Total cash flow hedge derivatives, net of tax	26	(81)	341	15
Total other comprehensive income (loss), net of tax	$ (5,853)	$ 14,496	$ (25,490)	$ 17,289