Whitney Acquisition - Additional Information (Detail) (USD $)	9 Months Ended			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 16, 2011 Whitney Bank Divested Branches [Member]	Sep. 30, 2012 Whitney [Member]	Sep. 16, 2011 Whitney [Member] Mississippi Gulf Coast [Member] Divestiture	Sep. 16, 2011 Whitney [Member] Bogalusa La [Member] Divestiture	Sep. 30, 2012 Common Stock
Business Acquisition [Line Items]
Shares in Hancock for each share of Whitney				0.418
TARP stock redeemed							$ 307,700,000
Business acquisition costs							1,600,000,000
Business acquisition, number of shares issued	40,794,261
Business acquisition, fair value of shares issued	1,300,000,000
Number of bank branches divested					7	1
Loans	11,451,820,000	11,191,901,000	47,000,000
Deposits	14,772,951,000	15,713,579,000	180,000,000
Business acquisition, purchase price allocation, total assets acquired	11,200,000,000
Business acquisition, purchase price allocation, loans and leases acquired	6,500,000,000
Business acquisition, purchase price allocation, investment securities acquired	2,600,000,000
Business acquisition, purchase price allocation, intangibles acquired	224,000,000
Business acquisition, purchase price allocation, liabilities assumed	10,100,000,000
Business Acquisition, Purchase Price Allocation, Deposits Assumed	9,200,000,000
Business Acquisition, Purchase Price Allocation, goodwill amount	589,000,000
Goodwill reduced for deferred tax purchase accounting adjustments	$ 22,200,000