Segment Reporting - Schedule of Segment Reporting Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Interest income	$ 189,205	$ 197,695	$ 571,410	$ 395,705
Interest expense	(11,949)	(20,653)	(40,407)	(52,840)
Net interest income	177,256	177,042	531,003	342,865
Provision for loan losses	(8,101)	(9,256)	(26,141)	(27,221)
Noninterest income	62,842	64,937	187,888	145,834
Depreciation and amortization	(8,067)	(6,383)	(25,229)	(13,738)
Other noninterest expense	(161,647)	(187,636)	(529,920)	(374,666)
Securities transactions	917	16	929	(71)
Income before income taxes	63,200	38,720	138,530	73,003
Income tax expense	16,216	8,342	33,747	15,210
Net income	46,984	30,378	104,783	57,793
Goodwill	628,877	629,688	628,877	629,688
Total assets	18,524,130	19,415,689	18,524,130	19,415,689	19,774,096
Total interest income from external customers	189,205	197,695	571,410	395,705
Hancock [Member]
Segment Reporting Information [Line Items]
Interest income	73,558	49,925	201,525	146,915
Interest expense	(5,402)	(9,230)	(17,587)	(31,262)
Net interest income	68,156	40,695	183,938	115,653
Provision for loan losses	(4,814)	(2,651)	(7,313)	(13,615)
Noninterest income	20,205	22,425	59,644	66,506
Depreciation and amortization	(3,830)	(2,062)	(10,884)	(6,572)
Other noninterest expense	(59,821)	(47,933)	(174,796)	(132,716)
Securities transactions	94		98	(51)
Income before income taxes	19,989	10,474	50,687	29,205
Income tax expense	5,433	2,220	11,808	4,790
Net income	14,556	8,254	38,879	24,415
Goodwill	94,130	23,386	94,130	23,386
Total assets	6,390,378	4,949,379	6,390,378	4,949,379
Total interest income from affiliates	1,010	1,080	3,033	3,347
Total interest income from external customers	72,548	48,845	198,492	143,568
Whitney [Member]
Segment Reporting Information [Line Items]
Interest income	110,917	144,049	355,776	237,907
Interest expense	(5,669)	(10,593)	(19,915)	(20,661)
Net interest income	105,248	133,456	335,861	217,246
Provision for loan losses	(2,353)	(6,278)	(18,696)	(11,981)
Noninterest income	31,938	37,471	98,122	65,356
Depreciation and amortization	(3,981)	(4,144)	(13,596)	(6,634)
Other noninterest expense	(90,087)	(133,643)	(320,985)	(223,755)
Securities transactions	823		824	20
Income before income taxes	41,588	26,862	81,530	40,252
Income tax expense	10,130	5,285	18,938	8,737
Net income	31,458	21,577	62,592	31,515
Goodwill	530,265	601,820	530,265	601,820
Total assets	12,343,043	14,121,103	12,343,043	14,121,103
Total interest income from affiliates	328	250	697	454
Total interest income from external customers	110,589	143,799	355,079	237,453
Other [Member]
Segment Reporting Information [Line Items]
Interest income	6,068	5,051	17,839	14,684
Interest expense	(2,100)	(2,044)	(6,290)	(4,372)
Net interest income	3,968	3,007	11,549	10,312
Provision for loan losses	(934)	(327)	(132)	(1,625)
Noninterest income	10,722	5,048	30,149	13,998
Depreciation and amortization	(256)	(177)	(749)	(532)
Other noninterest expense	(11,761)	(6,083)	(34,166)	(18,268)
Securities transactions		16	7	(40)
Income before income taxes	1,739	1,484	6,658	3,845
Income tax expense	653	837	3,001	1,683
Net income	1,086	647	3,657	2,162
Goodwill	4,482	4,482	4,482	4,482
Total assets	2,737,061	2,915,127	2,737,061	2,915,127
Total interest income from external customers	6,068	5,051	17,839	14,684
Eliminations [Member]
Segment Reporting Information [Line Items]
Interest income	(1,338)	(1,330)	(3,730)	(3,801)
Interest expense	1,222	1,214	3,385	3,455
Net interest income	(116)	(116)	(345)	(346)
Noninterest income	(23)	(7)	(27)	(26)
Other noninterest expense	22	23	27	73
Income before income taxes	(117)	(100)	(345)	(299)
Net income	(117)	(100)	(345)	(299)
Total assets	(2,946,352)	(2,569,920)	(2,946,352)	(2,569,920)
Total interest income from affiliates	$ (1,338)	$ (1,330)	$ (3,730)	$ (3,801)