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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, Before tax amount	$ 20,079	$ (9,275)	$ 38,903	$ (37,314)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	1,256	971	2,003	1,936
Total securities available for sale and transferred securities, Before tax amount	21,335	(8,304)	40,906	(35,378)
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	63	40	94	80
Change in net actuarial loss, Before tax amount	247	363	502	731
Total pension and other postretirement benefit plans, Before tax amount	372	465	721	936
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, Before tax amount	(101)	132	(164)	559
Reclassification adjustment for (gain) loss realized in net income, Before tax amount	(141)	(99)	(312)	(105)
Total cash flow hedge derivatives, Before tax amount	(242)	33	(476)	454
Total other comprehensive income (loss), Before tax amount	21,465	(7,806)	41,151	(33,988)
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, tax (expense) benefit	(5,020)	2,320	(9,726)	9,329
Change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(314)	(243)	(501)	(484)
Total securities available for sale and transferred securities, tax (expense) benefit	(5,334)	2,077	(10,227)	8,845
Pension and other postretirement benefit plans:
Net change in prior service costs, tax (expense) benefit	(15)	(16)	(31)	(32)
Recognized net loss due to lump sum settlements, tax (expense) benefit	(16)	(10)	(23)	(19)
Change in net actuarial loss, tax (expense) benefit	(61)	(91)	(126)	(183)
Total pension and other postretirement benefit plans, tax (expense) benefit	(92)	(117)	(180)	(234)
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, tax (expense) benefit	25	(33)	41	(140)
Reclassification adjustment for (gain) loss realized in net income, tax (expense) benefit	35	26	78	27
Total cash flow hedge derivatives, tax (expense) benefit	60	(7)	119	(113)
Total other comprehensive income (loss), tax (expense) benefit	(5,366)	1,953	(10,288)	8,498
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	15,059	(6,955)	29,177	(27,985)
Change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	942	728	1,502	1,452
Total securities available for sale and transferred securities, net of tax amount	16,001	(6,227)	30,679	(26,533)
Pension and other postretirement benefit plans:
Net change in prior service costs, net of tax amount	47	46	94	93
Recognized net loss due to lump sum settlements, net of tax amount	47	30	71	61
Change in net actuarial loss, net of tax amount	186	272	376	548
Total pension and other postretirement benefit plans, net of tax amount	280	348	541	702
Cash flow hedge derivatives
Change in accumulated gain (loss) on effective cash flow hedge derivatives, net of tax amount	(76)	99	(123)	419
Reclassification adjustment for (gain) loss realized in net income, net of tax amount	(106)	(73)	(234)	(78)
Total cash flow hedge derivatives, net of tax amount	(182)	26	(357)	341
Total other comprehensive income (loss), net of tax amount	$ 16,099	$ (5,853)	$ 30,863	$ (25,490)