Shareholders' Equity - Components of Accumulated Other Comprehensive Income (Loss) and the Related Tax Effects (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Securities available for sale and transferred securities, Before tax amount [Abstract]
Unrealized holding (losses) gains arising during the period, Before tax amount	$ (16,695)	$ 17,705	$ (67,135)
Reclassification adjustment for net gains realized in net income, Before tax amount	0	(300)	(485)
Change in net unrealized holding loss on securities transferred to held to maturity, Before tax amount	6,345	6,010	(46,383)
Total securities available for sale and transferred securities, Before tax amount	(10,350)	23,415	(114,003)
Securities available for sale and transferred securities, Tax (expense) benefit [Abstract]
Unrealized holding losses (gains) arising during the period, Tax (expense) benefit	6,386	(6,772)	25,679
Reclassification adjustment for net gains realized in net income, Tax (expense) benefit	0	115	186
Change in net unrealized holding loss on securities transferred to held to maturity, Tax (expense) benefit	(2,427)	(2,299)	17,741
Total securities available for sale and transferred securities, Tax (expense) benefit	3,959	(8,956)	43,606
Securities available for sale and transferred securities, Net of tax amount [Abstract]
Net unrealized holding (losses) gains on AFS Securities, net of tax at date of transfer	(10,309)	10,933	(41,456)
Less: adjustment for net gains realized in net income	0	(185)	(299)
Change in net unrealized holding loss on securities transferred to held to maturity, after tax income (expense)	3,918	3,711	(28,642)
Total securities available for sale and transferred securities, after tax income (expense)	(6,391)	14,459	(70,397)
Pension and other postretirement benefit plans, Before tax amount [Abstract]
Net change in prior service costs, before tax amount	250	249	251
Recognized net loss, before tax amount	2,221	905	2,225
Change in net actuarial loss, before tax amount	3,647	(21,657)	32,742
Total pension and other postretirement benefit plans, before tax amount	6,118	(19,148)	35,218
Pension and other postretirement benefit plans, Tax (expense) benefit [Abstract]
Net change in prior service costs, tax (expense) benefit	(96)	(95)	(96)
Recognized net loss, tax (expense) benefit	(850)	(346)	(851)
Change in net actuarial loss, tax (expense) benefit	(1,395)	8,283	(12,524)
Total pension and other postretirement benefit plans, tax (expense) benefit	2,341	(7,324)	13,471
Pension and other postretirement benefit plans, Net of tax amount [Abstract]
Net change in prior service costs	154	154	155
Recognized net loss, after tax income (expense)	1,371	559	1,374
Change in net actuarial loss, after tax income (expense)	2,252	(13,374)	20,218
Total pension and other postretirement benefit plans, after tax income (expense)	3,777	(11,824)	21,747
Cash flow hedge derivatives, Before tax amount [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives, before tax amount	(1,315)	(2,246)	2,468
Reclassification adjustment for loss realized in net income, before tax amount	836	(2)
Total cash flow hedge derivatives, before tax benefit	(479)	(2,248)
Total other comprehensive income (loss), Before tax amount	(4,711)	2,019	(76,317)
Cash flow hedge derivatives, Tax (expense) benefit [Abstract]
Change in accumulated gain (loss) on effective cash flow hedge derivatives, tax (expense) benefit	503	859	(944)
Reclassification adjustment for loss realized in net income, tax (expense) benefit	(320)	1
Total cash flow hedge derivatives, tax (expense) benefit	183	860
Total other comprehensive income (loss), Tax expense (benefit)	1,801	(772)	29,191
Cash flow hedge derivatives, Net of tax amount [Abstract]
Change in the accumulated gain (loss) on effective cash flow hedge derivatives, after tax amount	(812)	(1,387)	1,524
Less: adjustment for loss realized in net income	516	(1)	0
Total cash flow hedge derivatives, after tax amount	(296)	(1,388)
Total other comprehensive income (loss), Net of tax	$ (2,910)	1,247	$ (47,126)
BancTrust Financial Group, Inc. [Member]
Pension and other postretirement benefit plans, Before tax amount [Abstract]
Recognized net loss, before tax amount		1,355
Pension and other postretirement benefit plans, Tax (expense) benefit [Abstract]
Recognized net loss, tax (expense) benefit		(518)
Pension and other postretirement benefit plans, Net of tax amount [Abstract]
Recognized net loss, after tax income (expense)		$ 837