Segment Reporting - Schedule of Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Interest income	$ 185,272	$ 191,716
Interest expense	11,257	15,428
Net interest income	174,015	176,288
Provision for loan losses	(9,578)	(10,015)
Noninterest income	60,187	61,494
Depreciation and amortization	(7,961)	(8,695)
Other noninterest expense	(151,641)	(196,768)
Securitites transactions		12
Income before income taxes	65,022	22,316
Income tax expense	16,446	3,821
Net income	48,576	18,495
Goodwill	625,675	647,216
Total assets	19,064,123	19,291,097	19,464,485
Total interest income from external customers	185,272	191,716
Hancock [Member]
Segment Reporting Information [Line Items]
Interest income	62,813	53,243
Interest expense	(4,943)	(6,540)
Net interest income	57,870	46,703
Provision for loan losses	(5,422)	2,165
Noninterest income	18,411	18,816
Depreciation and amortization	(3,698)	(3,287)
Other noninterest expense	(52,975)	(51,878)
Securitites transactions		4
Income before income taxes	14,186	12,523
Income tax expense	2,690	535
Net income	11,496	11,988
Goodwill	94,130	94,130
Total assets	6,380,941	6,331,361
Total interest income from affiliates	992	908
Total interest income from external customers	61,821	52,335
Whitney [Member]
Segment Reporting Information [Line Items]
Interest income	117,799	133,749
Interest expense	(5,157)	(7,846)
Net interest income	112,642	125,903
Provision for loan losses	(2,980)	(13,641)
Noninterest income	30,794	33,691
Depreciation and amortization	(3,981)	(5,185)
Other noninterest expense	(86,863)	(134,113)
Securitites transactions		1
Income before income taxes	49,612	6,656
Income tax expense	12,943	1,515
Net income	36,669	5,141
Goodwill	527,063	548,604
Total assets	12,696,705	12,751,548
Total interest income from affiliates	215
Total interest income from external customers	117,584	133,749
Other [Member]
Segment Reporting Information [Line Items]
Interest income	5,867	5,000
Interest expense	(2,249)	(1,203)
Net interest income	3,618	3,797
Provision for loan losses	(1,176)	1,461
Noninterest income	10,995	8,989
Depreciation and amortization	(282)	(223)
Other noninterest expense	(11,816)	(10,779)
Securitites transactions		7
Income before income taxes	1,339	3,252
Income tax expense	813	1,771
Net income	526	1,481
Goodwill	4,482	4,482
Total assets	2,884,581	2,683,298
Total interest income from external customers	5,867	5,000
Eliminations [Member]
Segment Reporting Information [Line Items]
Interest income	(1,207)	(276)
Interest expense	1,092	161
Net interest income	(115)	(115)
Noninterest income	(13)	(2)
Other noninterest expense	13	2
Income before income taxes	(115)	(115)
Net income	(115)	(115)
Total assets	(2,898,104)	(2,475,110)
Total interest income from affiliates	(1,207)	(908)
Total interest income from external customers		$ 632