Segment Reporting - Schedule of Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Interest income	$ 179,649	$ 190,489	$ 364,921	$ 382,205
Interest expense	(10,470)	(13,030)	(21,727)	(28,458)
Net interest income	169,179	177,459	343,194	353,747
Provision for loan losses	8,257	8,025	17,835	18,040
Noninterest income	63,897	63,552	124,084	125,046
Depreciation and amortization	(8,060)	(8,466)	(16,021)	(17,159)
Other noninterest expense	(154,190)	(171,506)	(305,831)	(368,276)
Securities transactions				12
Income before income taxes	62,569	53,014	127,591	75,330
Income tax expense	15,707	13,710	32,153	17,531
Net income	46,862	39,304	95,438	57,799
Goodwill	625,675	628,877	625,675	628,877	628,877
Total assets	18,934,301	18,778,707	18,934,301	18,778,707	19,464,485
Total interest income from external customers	179,649	190,489	364,921	382,205
Hancock [Member]
Segment Reporting Information [Line Items]
Interest income	68,793	74,725	131,606
Interest expense	(4,492)	(5,645)	(9,435)
Net interest income	64,301	69,080	122,171
Provision for loan losses	(3,085)	(4,664)	(8,507)
Noninterest income	19,866	20,622	38,277
Depreciation and amortization	(3,891)	(3,767)	(7,589)
Other noninterest expense	(54,306)	(63,096)	(107,281)
Securities transactions			4
Income before income taxes	22,885	18,175	37,071
Income tax expense	4,493	5,840	7,183
Net income	18,392	12,335	29,888
Goodwill	94,130	94,130	94,130	94,130
Total assets	6,562,468	6,448,429	6,562,468	6,448,429
Total interest income from affiliates	942	884	1,934
Total interest income from external customers	67,851	73,841	129,672
Whitney [Member]
Segment Reporting Information [Line Items]
Interest income	105,614	111,110	223,413	244,859
Interest expense	(4,420)	(6,400)	(9,579)	(14,246)
Net interest income	101,194	104,710	213,834	230,613
Provision for loan losses	(3,935)	(2,702)	(6,915)	(16,343)
Noninterest income	32,118	32,493	62,912	66,184
Depreciation and amortization	(3,849)	(4,430)	(7,830)	(9,615)
Other noninterest expense	(86,538)	(96,784)	(173,401)	(230,897)
Securities transactions				1
Income before income taxes	38,990	33,287	88,600	39,943
Income tax expense	10,571	7,293	23,514	8,808
Net income	28,419	25,994	65,086	31,135
Goodwill	527,063	530,265	527,063	530,265
Total assets	12,589,429	12,426,207	12,589,429	12,426,207
Total interest income from affiliates	225	208	440	369
Total interest income from external customers	105,389	110,902	222,973	244,490
Other [Member]
Segment Reporting Information [Line Items]
Interest income	6,409	5,746	12,276	11,539
Interest expense	(2,610)	(1,962)	(4,857)	(3,958)
Net interest income	3,799	3,784	7,419	7,581
Provision for loan losses	(1,237)	(659)	(2,413)	802
Noninterest income	11,924	10,437	22,919	19,427
Depreciation and amortization	(320)	(269)	(602)	(490)
Other noninterest expense	(13,357)	(11,626)	(25,173)	(22,408)
Securities transactions				7
Income before income taxes	809	1,667	2,150	4,919
Income tax expense	643	577	1,456	2,348
Net income	166	1,090	694	2,571
Goodwill	4,482	4,482	4,482	4,482
Total assets	2,747,840	2,709,431	2,747,840	2,709,431
Total interest income from external customers	6,409	5,746	12,276	11,539
Eliminations [Member]
Segment Reporting Information [Line Items]
Interest income	(1,167)	(1,092)	(2,374)	(2,161)
Interest expense	1,052	977	2,144	1,931
Net interest income	(115)	(115)	(230)	(230)
Noninterest income	(11)		(24)	(3)
Other noninterest expense	11		24	3
Income before income taxes	(115)	(115)	(230)	(230)
Net income	(115)	(115)	(230)	(230)
Total assets	(2,965,436)	(2,805,360)	(2,965,436)	(2,805,360)
Total interest income from affiliates	(1,167)	(1,092)	(2,374)	(2,161)
Total interest income from external customers