Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Securities available for sale and transferred securities, Before tax amount [Abstract]
Unrealized holding (losses) gains arising during the period, Before tax amount	$ (15,656)	$ (16,695)	$ 17,705
Reclassification adjustment for net losses (gains) realized in net income, Before tax amount	310	0	(300)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	9,830	6,345	6,010
Total securities available for sale and transferred securities, Before tax amount	(5,516)	(10,350)	23,415
Securities available for sale and transferred securities, Tax (expense) benefit [Abstract]
Unrealized holding (losses) gains arising during the period, Tax (expense) benefit	5,989	6,386	(6,772)
Reclassification adjustment for net gains realized in net income, Tax (expense) benefit	(119)	0	115
Change in net unrealized holding loss on securities transferred to held to maturity, Tax (expense) benefit	(3,760)	(2,427)	(2,299)
Total securities available for sale and transferred securities, Tax (expense) benefit	2,110	3,959	(8,956)
Securities available for sale and transferred securities, Net of tax amount [Abstract]
Unrealized holding (losses) gains arising during the period, net of tax amount	(9,667)	(10,309)	10,933
Reclassification adjustment for net losses (gains) realized in net income, net of tax amount	191	0	(185)
Change in net unrealized holding losses on securities transferred to held to maturity	6,070	3,918	3,711
Total securities available for sale and transferred securities, net of tax amount	(3,406)	(6,391)	14,459
Pension and other postretirement benefit plans, Before tax amount [Abstract]
Net change in prior service costs, before tax amount	250	250	249
Recognized net loss due to BancTrust termination, before tax amount			1,355
Recognized net loss due to lump sum settlements, before tax amount	3,906	2,221	905
Change in net actuarial loss, before tax amount	476	3,647	(21,657)
Total pension and other postretirement benefit plans, before tax amount	4,632	6,118	(19,148)
Pension and other postretirement benefit plans, Tax (expense) benefit [Abstract]
Net change in prior service costs, tax (expense) benefit	(96)	(96)	(95)
Recognized net loss due to BancTrust termination, tax (expense) benefit			(518)
Recognized net loss due to lump sum settlements, tax (expense) benefit	(1,494)	(850)	(346)
Change in net actuarial loss, tax (expense) benefit	(182)	(1,395)	8,283
Total pension and other postretirement benefit plans, tax (expense) benefit	1,772	2,341	(7,324)
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 BancTrust termination, net of tax amount			837
Recognized net loss due to lump sum settlements, net of tax amount	2,412	1,371	559
Change in net actuarial loss, net of tax amount	294	2,252	(13,374)
Total pension and other postretirement benefit plans, net of tax amount	2,860	3,777	(11,824)
Cash flow hedge derivatives, Before tax amount [Abstract]
Change in accumulated loss on effective cash flow hedge derivatives, before tax amount	(369)	(1,315)	(2,246)
Reclassification adjustment for loss realized in net income, before tax amount	599	836	(2)
Total cash flow hedge derivatives, before tax benefit	230	(479)	(2,248)
Total other comprehensive income (loss), Before tax amount	(654)	(4,711)	2,019
Cash flow hedge derivatives, Tax (expense) benefit [Abstract]
Change in accumulated loss on effective cash flow hedge derivatives, tax (expense) benefit	141	503	859
Reclassification adjustment for loss realized in net income, tax (expense) benefit	(229)	(320)	1
Total cash flow hedge derivatives, tax (expense) benefit	(88)	183	860
Total other comprehensive income (loss), Tax expense (benefit)	250	1,801	(772)
Cash flow hedge derivatives, Net of tax amount [Abstract]
Change in the accumulated loss on effective cash flow hedge derivatives, net of tax amount	(228)	(812)	(1,387)
Reclassification adjustment for loss realized in net income, net of tax amount	370	516	(1)
Total cash flow hedge derivatives, net of tax amount	142	(296)	(1,388)
Total other comprehensive income (loss), net of tax amount	$ (404)	$ (2,910)	$ 1,247