Shareholders' Equity - Net Change in Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, Before tax amount	$ 18,824	$ (28,039)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	747	965
Total securities available for sale and transferred securities, Before tax amount	19,571	(27,074)
Pension and other postretirement benefit plans:
Net change in prior service costs, Before tax amount	63	63
Recognized net loss due to lump sum settlements, Before tax amount	31	40
Change in net actuarial loss, Before tax amount	255	368
Total pension and other postretirement benefit plans, Before tax amount	349	471
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, Before tax amount	(63)	427
Reclassification adjustment for (gain) loss realized in net income, Before tax amount	(171)	(6)
Total cash flow hedge derivatives, Before tax amount	(234)	421
Total other comprehensive income (loss), Before tax amount	19,686	(26,182)
Securities available for sale and transferred securities:
Net unrealized holding gains (losses) arising during the period, tax (expense) benefit	(4,706)	7,009
Change in net unrealized holding loss on securities transferred to held to maturity, tax (expense) benefit	(187)	(241)
Total securities available for sale and transferred securities, tax (expense) benefit	(4,893)	6,768
Pension and other postretirement benefit plans:
Net change in prior service costs, tax (expense) benefit	(16)	(16)
Recognized net loss due to lump sum settlements, tax (expense) benefit	(7)	(9)
Change in net actuarial loss, tax (expense) benefit	(65)	(92)
Total pension and other postretirement benefit plans, tax (expense) benefit	(88)	(117)
Cash flow hedge derivatives:
Change in accumulated gain (loss) on effective cash flow hedge derivatives, tax (expense) benefit	16	(107)
Reclassification adjustment for (gain) loss realized in net income, tax (expense) benefit	43	1
Total cash flow hedge derivatives, tax (expense) benefit	59	(106)
Total other comprehensive income (loss), tax (expense) benefit	(4,922)	6,545
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	14,118	(21,030)
Change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	560	724
Total securities available for sale and transferred securities, net of tax amount	14,678	(20,306)
Pension and other postretirement benefit plans:
Net change in prior service costs, net of tax amount	47	47
Recognized net loss due to lump sum settlements, net of tax amount	24	31
Change in net actuarial loss, net of tax amount	190	276
Total pension and other postretirement benefit plans, net of tax amount	261	354
Cash flow hedge derivatives
Change in accumulated gain (loss) on effective cash flow hedge derivatives, net of tax amount	(47)	320
Reclassification adjustment for (gain) loss realized in net income, net of tax amount	(128)	(5)
Total cash flow hedge derivatives, net of tax amount	(175)	315
Total other comprehensive income (loss), net of tax amount	$ 14,764	$ (19,637)