Mergers and Acquisitions - (Details) $ / shares in Units, $ in Thousands		Jul. 01, 2015 USD ($) branch $ / shares shares	Sep. 01, 2013 USD ($) branch shares	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Aug. 31, 2011 USD ($) trust	Feb. 04, 2011 USD ($) branch	Jul. 23, 2010 USD ($) branch
Business Acquisition [Line Items]
Goodwill resulting from merger				$ 452,037	$ 274,706	$ 274,658
Intangible assets (excluding goodwill)				$ 30,562	$ 22,624
Eligible losses covered (percent)				80.00%
Eligible recoveries covered (percent)				80.00%
Heritage Financial Group
Business Acquisition [Line Items]
Total Purchase Price		$ 297,260
Payment to Heritage stock option holders		$ 5,915
Voting interest acquired (percent)		100.00%
Number of banking, mortgage, and investment offices | branch		48
Cash paid for each share of the acquired entity's shares, factor in excess of exercise price (usd per share) | $ / shares		$ 27.00
Intangible assets, including goodwill		$ 189,589
Goodwill resulting from merger	[1]	177,333
Intangible assets (excluding goodwill)	[1]	12,256
Borrowings		$ 314,656
Heritage Financial Group | Core Deposits
Business Acquisition [Line Items]
Weighted average useful life		10 years
Heritage Financial Group | Common Stock
Business Acquisition [Line Items]
Shares issued to common shareholders | shares		8,635,879
Number of shares for each share of acquired entity's shares		0.9266
First M&F Merger
Business Acquisition [Line Items]
Number of branches in operation | branch			43
Total Purchase Price			$ 157
Voting interest acquired (percent)			100.00%
Intangible assets, including goodwill			$ 115
Goodwill resulting from merger			$ 90
First M&F Merger | Core Deposits
Business Acquisition [Line Items]
Weighted average useful life			10 years
Intangible assets, other than goodwill			$ 25,032
First M&F Merger | Junior Subordinated Debt
Business Acquisition [Line Items]
Borrowings			$ 31
Variable interest rate of derivatives, description			90-day LIBOR
Fair value adjustment			$ 12
First M&F Merger | LIBOR | Junior Subordinated Debt
Business Acquisition [Line Items]
Basis spread on variable rate (percent)			1.33%
First M&F Merger | Common Stock
Business Acquisition [Line Items]
Shares issued to common shareholders | shares			6,175,576
Royal Bank Client
Business Acquisition [Line Items]
Assets under management							$ 680,000
Trust transferred under acquisition (trust) | trust							200
American Trust Bank
Business Acquisition [Line Items]
Number of branches in the northwest region of Georgia (branches) | branch								3
Value of loans covered in loss-share agreements with the FDIC								$ 73,657
Eligible losses covered (percent)								80.00%
Eligible recoveries covered (percent)								80.00%
Crescent Bank & Trust Company
Business Acquisition [Line Items]
Number of branches in the northwest region of Georgia (branches) | branch									11
Value of loans covered in loss-share agreements with the FDIC									$ 361,472
Other real estate owned									$ 50,168
Eligible losses covered (percent)									80.00%

[1] The goodwill resulting from the merger has been assigned to the Community Banks operating segment.