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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, Before tax amount	$ (9,660)	$ (535)	$ (46,974)	$ 6,335
Reclassification adjustment for net (gains) losses realized in net income, Before tax amount	0	(14)	0	(15)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	973	1,130	2,909	3,551
Total securities available for sale and transferred securities, Before tax amount	(8,687)	581	(44,065)	9,871
Pension and other postretirement benefit plans:
Net change in prior service costs, Before tax amount	63	63	188	188
Recognized net loss due to lump sum settlements, Before tax amount	41	0	121	0
Change in net actuarial loss, Before tax amount	361	356	1,092	1,923
Recognized net loss due to defined benefit plan termination, Before tax amount			0	16,991
Total pension and other postretirement benefit plans, Before tax amount	465	419	1,401	19,102
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, Before tax amount	52	53	611	(99)
Reclassification adjustment for (gain) loss realized in net income, Before tax amount	(104)	55	(209)	232
Total cash flow hedge derivatives, Before tax amount	(52)	108	402	133
Total other comprehensive income (loss), Before tax amount	(8,274)	1,108	(42,262)	29,106
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, tax (expense) benefit	2,414	204	11,743	(2,423)
Reclassification adjustment for net (gains) losses realized in net income, tax (expense) benefit	0	(6)	0	(6)
Change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(243)	(432)	(727)	(1,358)
Total securities available for sale and transferred securities, tax (expense) benefit	2,171	(222)	11,016	(3,775)
Reclassification adjustment for net (gains) losses realized in net income, tax (expense) benefit	0	6	0	6
Pension and other postretirement benefit plans:
Net change in prior service costs, tax (expense) benefit	(16)	(24)	(48)	(72)
Recognized net loss due to lump sum settlements, tax (expense) benefit	(10)	0	(29)	0
Change in net actuarial loss, tax (expense) benefit	(90)	(137)	(273)	(736)
Recognized net loss due to defined benefit plan termination, tax (expense) benefit			0	(6,499)
Total pension and other postretirement benefit plans, tax (expense) benefit	(116)	(161)	(350)	(7,307)
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, tax (expense) benefit	(13)	(20)	(153)	38
Reclassification adjustment for (gain) loss realized in net income, tax (expense) benefit	25	(21)	52	(89)
Total cash flow hedge derivatives, tax (expense) benefit	12	(41)	(101)	(51)
Total other comprehensive income (loss), tax (expense) benefit	2,067	(424)	10,565	(11,133)
Net unrealized gains (losses) on available for sale securities and transferred securities:
Net unrealized holding gains (losses) arising during the period, net of tax amount	(7,246)	(331)	(35,231)	3,912
Reclassification adjustment for net (gains) losses realized in net income, net of tax amount	0	(8)	0	(9)
Change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	730	698	2,182	2,193
Total securities available for sale and transferred securities, net of tax amount	(6,516)	359	(33,049)	6,096
Pension and other postretirement benefit plans:
Net change in prior service costs, net of tax amount	47	39	140	116
Recognized net loss due to lump sum settlements, net of tax amount	31	0	92	0
Change in net actuarial loss, net of tax amount	271	219	819	1,187
Recognized net loss due to defined benefit plan termination, net of tax amount	0	0	0	10,492
Total pension and other postretirement benefit plans, net of tax amount	349	258	1,051	11,795
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, net of tax amount	39	33	458	(61)
Reclassification adjustment for (gain) loss realized in net income, net of tax amount	(79)	34	(157)	143
Total cash flow hedge derivatives, net of tax amount	(40)	67	301	82
Total other comprehensive income (loss), net of tax amount	$ (6,207)	$ 684	$ (31,697)	$ 17,973