Loans and Allowance for Loan Losses - Allowance for Loan Losses by Portfolio Segment and Related Recorded Investment in Loans (Detail) (USD $) In Thousands, unless otherwise specified			9 Months Ended																																								9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Allowance for loan losses [Member]	Sep. 30, 2011 Allowance for loan losses [Member]		Sep. 30, 2012 Allowance for loan losses [Member] Commercial loans [Member]	Sep. 30, 2011 Allowance for loan losses [Member] Commercial loans [Member]		Sep. 30, 2012 Allowance for loan losses [Member] Residential mortgage loans [Member]	Sep. 30, 2011 Allowance for loan losses [Member] Residential mortgage loans [Member]		Sep. 30, 2012 Allowance for loan losses [Member] Consumer loans [Member]	Sep. 30, 2011 Allowance for loan losses [Member] Consumer loans [Member]		Sep. 30, 2012 Loans [Member]	Sep. 30, 2011 Loans [Member]		Sep. 30, 2012 Loans [Member] Commercial loans [Member]	Sep. 30, 2011 Loans [Member] Commercial loans [Member]		Sep. 30, 2012 Loans [Member] Residential mortgage loans [Member]	Sep. 30, 2011 Loans [Member] Residential mortgage loans [Member]		Sep. 30, 2012 Loans [Member] Consumer loans [Member]	Sep. 30, 2011 Loans [Member] Consumer loans [Member]		Sep. 30, 2012 Originated loans [Member] Allowance for loan losses [Member]	Sep. 30, 2011 Originated loans [Member] Allowance for loan losses [Member]	Sep. 30, 2012 Originated loans [Member] Allowance for loan losses [Member] Commercial loans [Member]	Sep. 30, 2011 Originated loans [Member] Allowance for loan losses [Member] Commercial loans [Member]	Sep. 30, 2012 Originated loans [Member] Allowance for loan losses [Member] Residential mortgage loans [Member]	Sep. 30, 2011 Originated loans [Member] Allowance for loan losses [Member] Residential mortgage loans [Member]	Sep. 30, 2012 Originated loans [Member] Allowance for loan losses [Member] Consumer loans [Member]	Sep. 30, 2011 Originated loans [Member] Allowance for loan losses [Member] Consumer loans [Member]	Sep. 30, 2012 Originated loans [Member] Loans [Member]	Sep. 30, 2011 Originated loans [Member] Loans [Member]	Sep. 30, 2012 Originated loans [Member] Loans [Member] Commercial loans [Member]	Sep. 30, 2011 Originated loans [Member] Loans [Member] Commercial loans [Member]	Sep. 30, 2012 Originated loans [Member] Loans [Member] Residential mortgage loans [Member]	Sep. 30, 2011 Originated loans [Member] Loans [Member] Residential mortgage loans [Member]	Sep. 30, 2012 Originated loans [Member] Loans [Member] Consumer loans [Member]	Sep. 30, 2011 Originated loans [Member] Loans [Member] Consumer loans [Member]	Sep. 30, 2012 Covered loans [Member] Allowance for loan losses [Member]	Sep. 30, 2011 Covered loans [Member] Allowance for loan losses [Member]	Sep. 30, 2012 Covered loans [Member] Allowance for loan losses [Member] Commercial loans [Member]	Sep. 30, 2011 Covered loans [Member] Allowance for loan losses [Member] Commercial loans [Member]	Sep. 30, 2012 Covered loans [Member] Allowance for loan losses [Member] Residential mortgage loans [Member]	Sep. 30, 2012 Covered loans [Member] Allowance for loan losses [Member] Consumer loans [Member]	Sep. 30, 2011 Covered loans [Member] Allowance for loan losses [Member] Consumer loans [Member]	Sep. 30, 2012 Covered loans [Member] Loans [Member]	Sep. 30, 2011 Covered loans [Member] Loans [Member]	Sep. 30, 2012 Covered loans [Member] Loans [Member] Commercial loans [Member]	Sep. 30, 2011 Covered loans [Member] Loans [Member] Commercial loans [Member]	Sep. 30, 2012 Covered loans [Member] Loans [Member] Residential mortgage loans [Member]	Sep. 30, 2011 Covered loans [Member] Loans [Member] Residential mortgage loans [Member]	Sep. 30, 2012 Covered loans [Member] Loans [Member] Consumer loans [Member]	Sep. 30, 2011 Covered loans [Member] Loans [Member] Consumer loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases receivable, Allowance, Beginning Balance	$ 135,591	$ 124,881	$ 124,881	$ 81,997		$ 78,414	$ 56,859		$ 13,918	$ 4,626		$ 32,549	$ 20,512														$ 83,246	$ 81,325	$ 60,211	$ 56,859	$ 4,894	$ 4,626	$ 18,141	$ 19,840	$ 6,581,194	$ 4,532,720	$ 4,465,736	$ 3,119,291	$ 757,471	$ 412,267	$ 1,357,987	$ 1,001,162	$ 41,635	$ 672	$ 18,203		$ 9,024	$ 14,408	$ 672	$ 555,732	$ 721,831	$ 176,724	$ 301,960	$ 271,618	$ 262,246	$ 107,390	$ 157,625
Charge-offs			(57,427)	(36,602)		(40,875)	(25,598)		(4,889)	(1,774)		(11,663)	(9,230)														(34,588)	(36,227)	(18,036)	(25,598)	(4,889)	(1,774)	(11,663)	(8,855)									(22,839)	(375)	(22,839)				(375)
Recoveries			7,595	13,720		4,225	9,863		310	1,044		3,060	2,813														7,595	13,720	4,225	9,863	310	1,044	3,060	2,813
Net provision for loan losses			26,141	27,221	[1]	25,362	18,842	[1]	6,532	5,286	[1]	(5,753)	3,093	[1]													23,496	25,548	12,618	17,514	4,336	5,286	6,542	2,748									2,645	1,673	12,744	1,328	2,196	(12,295)	345
Increase in indemnification asset			34,401	31,777	[1]	22,650	19,583	[1]	11,189			562	12,194	[1]																													34,401	31,777	22,650	19,583	11,189	562	12,194
Loans and Leases receivable, Allowance, Ending Balance	135,591	124,881	135,591	118,113		89,776	79,549		27,060	9,182		18,755	29,382		11,434,448	11,102,269		8,187,469	8,075,247		1,561,640	1,451,506		1,685,339	1,575,516		79,749	84,366	59,018	58,638	4,651	9,182	16,080	16,546	6,581,194	4,532,720	4,465,736	3,119,291	757,471	412,267	1,357,987	1,001,162	55,842	33,747	30,758	20,911	22,409	2,675	12,836	555,732	721,831	176,724	301,960	271,618	262,246	107,390	157,625
Ending balance: Individually evaluated for impairment			8,602	6,600		8,469	5,959		133	641					92,023	83,622		84,106	77,186		7,917	6,436					8,602	6,600	8,469	5,959	133	641			85,861	49,516	78,337	44,317	7,524	5,199										6,162	34,106	5,769	32,869	393	1,237
Ending balance: Collectively evaluated for impairment			126,989	111,513		81,307	73,590		26,927	8,541		18,755	29,382		11,342,425	11,018,647		8,103,363	7,998,061		1,553,723	1,445,070		1,685,339	1,575,516		71,147	77,766	50,549	52,679	4,518	8,541	16,080	16,546	6,495,333	4,483,204	4,387,399	3,074,974	749,947	407,068	1,357,987	1,001,162	55,842	33,747	30,758	20,911	22,409	2,675	12,836	549,570	687,725	170,955	269,091	271,225	261,009	107,390	157,625
Acquired loans															$ 4,297,522	$ 5,847,718	[2]	$ 3,545,009	$ 4,653,996	[2]	$ 532,551	$ 776,993	[2]	$ 219,962	$ 416,729	[2]

[1] (a) The Company increased the allowance by $33.4 million for losses related to impairment on certain pools of covered loans. This provision was mostly offset by a $31.8 million increase in the FDIC indemnification asset.

[2] In accordance with purchase accounting rules, the Whitney loans were recorded at their fair value at the time of the acquisition, and the prior allowance for loan losses was eliminated. No allowance has been established on these acquired loans since the acquisition date. These loans are included in the ending balance of loans collectively evaluated for impairment.