Goodwill and Identifiable Intangible Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill [Roll Forward]
Goodwill, beginning of period	$ 372,851,000	$ 291,104,000
Goodwill from acquisitions		81,747,000
Measurement period adjustment during 2014	(7,351,000)
Balance, end of period	365,500,000	372,851,000	291,104,000
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	99,532,000	99,532,000
Accumulated Amortization	66,298,000	57,542,000
Net Carrying Amount	33,234,000	41,990,000
Remaining Weighted-Average Amortization Period in Years	7 years 4 months 24 days
Amortization expense of identifiable intangible assets	8,800,000	8,800,000	3,800,000
Future amortization expense for identifiable intangible assets [Abstract]
2015	7,600,000
2016	6,700,000
2017	5,700,000
2018	4,800,000
2019	3,600,000
Impairment losses on identifiable intangible assets	0	0	0
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	85,824,000	85,824,000
Accumulated Amortization	54,464,000	46,437,000
Net Carrying Amount	31,360,000	39,387,000
Remaining Weighted-Average Amortization Period in Years	7 years 6 months
Insurance Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	11,693,000	11,693,000
Accumulated Amortization	10,566,000	9,967,000
Net Carrying Amount	1,127,000	1,726,000
Remaining Weighted-Average Amortization Period in Years	4 years 10 months 24 days
Banking Charters [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	1,325,000	1,325,000
Accumulated Amortization	745,000	678,000
Net Carrying Amount	580,000	647,000
Remaining Weighted-Average Amortization Period in Years	8 years 8 months 12 days
Borrower Relationship Intangible [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	690,000	690,000
Accumulated Amortization	523,000	460,000
Net Carrying Amount	167,000	230,000
Remaining Weighted-Average Amortization Period in Years	2 years 8 months 12 days
General Banking [Member]
Goodwill [Roll Forward]
Goodwill, beginning of period	328,483,000	246,736,000
Goodwill from acquisitions		81,747,000
Measurement period adjustment during 2014	(7,351,000)
Balance, end of period	321,132,000	328,483,000
Insurance [Member]
Goodwill [Roll Forward]
Goodwill, beginning of period	44,368,000	44,368,000
Goodwill from acquisitions		0
Measurement period adjustment during 2014	0
Balance, end of period	44,368,000	44,368,000
BancTrust acquisition [Member]
Goodwill [Roll Forward]
Goodwill acquisition		81,600,000
SOUTHBank Branches [Member]
Goodwill [Roll Forward]
Goodwill acquisition		$ 150,000
Number of branches		2