Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities available for sale and transferred securities, Before tax amount [Abstract]
Unrealized holding gains (losses) arising during the period, Before tax amount	$ (13,994)	$ (15,656)	$ (16,695)
Reclassification adjustment for net (gains) losses realized in net income, Before tax amount	(15)	310	0
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	4,721	9,830	6,345
Total securities available for sale and transferred securities, Before tax amount	(9,288)	(5,516)	(10,350)
Securities available for sale and transferred securities, Tax (expense) benefit [Abstract]
Unrealized holding gains (losses) arising during the period, Tax (expense) benefit	5,353	5,989	6,386
Reclassification adjustment for net (gains) losses realized in net income, Tax (expense) benefit	6	(119)	0
Change in net unrealized holding loss on securities transferred to held to maturity, Tax (expense) benefit	(1,806)	(3,760)	(2,427)
Total securities available for sale and transferred securities, Tax (expense) benefit	3,553	2,110	3,959
Securities available for sale and transferred securities, Net of tax amount [Abstract]
Unrealized holding gains (losses) arising during the period, net of tax amount	(8,641)	(9,667)	(10,309)
Reclassification adjustment for net (gains) losses realized in net income, net of tax amount	(9)	191	0
Change in net unrealized holding loss on securities transferred to held to maturity, net of tax amount	2,915	6,070	3,918
Total securities available for sale and transferred securities, net of tax amount	(5,735)	(3,406)	(6,391)
Pension and other postretirement benefit plans, Before tax amount [Abstract]
Net change in prior service costs, before tax amount	250	250	250
Recognized net loss due to lump sum settlements, before tax amount		3,906	2,221
Change in net actuarial loss, before tax amount	731	476	3,647
Recognized net loss due to defined benefit plan termination, before tax amount	17,662
Total pension and other postretirement benefit plans, before tax amount	18,643	4,632	6,118
Pension and other postretirement benefit plans, Tax (expense) benefit [Abstract]
Net change in prior service costs, tax (expense) benefit	(96)	(96)	(96)
Recognized net loss due to lump sum settlements, tax (expense) benefit		(1,494)	(850)
Change in net actuarial loss, tax (expense) benefit	(280)	(182)	(1,395)
Recognized net loss due to defined benefit plan termination, tax (expense) benefit	(6,755)
Total pension and other postretirement benefit plans, tax (expense) benefit	(7,131)	(1,772)	(2,341)
Pension and other postretirement benefit plans, Net of tax amount [Abstract]
Net change in prior service costs, net of tax amount	154	154	154
Recognized net loss due to lump sum settlements, net of tax amount	0	2,412	1,371
Change in net actuarial loss, net of tax amount	451	294	2,252
Recognized net loss due to defined benefit plan termination, net of tax	10,907	0	0
Total pension and other postretirement benefit plans, net of tax amount	11,512	2,860	3,777
Cash flow hedge derivatives, Before tax amount [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives, before tax amount	198	(369)	(1,315)
Reclassification adjustment for loss realized in net income, before tax amount	282	599	836
Total cash flow hedge derivatives, before tax benefit	480	230	(479)
Total other comprehensive income (loss), Before tax amount	9,835	(654)	(4,711)
Cash flow hedge derivatives, Tax (expense) benefit [Abstract]
Change in accumulated loss on effective cash flow hedge derivatives, tax (expense) benefit	(76)	141	503
Reclassification adjustment for loss realized in net income, tax (expense) benefit	(108)	(229)	(320)
Total cash flow hedge derivatives, tax (expense) benefit	(184)	(88)	183
Total other comprehensive income (loss), Tax expense (benefit)	(3,762)	250	1,801
Cash flow hedge derivatives, Net of tax amount [Abstract]
Change in the accumulated loss on effective cash flow hedge derivatives, net of tax amount	122	(228)	(812)
Reclassification adjustment for loss realized in net income, net of tax amount	174	370	516
Total cash flow hedge derivatives, net of tax amount	296	142	(296)
Total other comprehensive income (loss), net of tax amount	$ 6,073	$ (404)	$ (2,910)