Fair Value - Schedule of Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	$ 2,208,789	$ 4,496,900
Held to maturity securities	1,889,136
Loans, net	11,298,857	11,052,145
Loans held for sale	51,097	72,378
Deposits	14,772,951	15,713,579
Long-term debt	308,327	353,890
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	734,680	1,622,366
Available for sale securities	27,441	260,814
Accrued interest receivable	47,607	53,973
Federal funds purchased	29,521	16,819
Securities sold under agreements to repurchase	719,113	1,027,635
Accrued interest payable	6,526	8,284
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	2,181,348	4,236,086
Held to maturity securities	1,889,136
Loans, net	77,259	55,252
Loans held for sale	50,389	72,378
Derivative financial instruments	21,198	14,952
Long-term debt	333,371	365,421
Derivative financial instruments	22,423	15,643
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, net	11,484,291	11,134,410
Deposits	14,789,902	15,737,667
Total Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	734,680	1,622,366
Available for sale securities	2,208,789	4,496,900
Held to maturity securities	1,889,136
Loans, net	11,561,550	11,189,662
Loans held for sale	50,389	72,378
Accrued interest receivable	47,607	53,973
Derivative financial instruments	21,198	14,952
Deposits	14,789,902	15,737,667
Federal funds purchased	29,521	16,819
Securities sold under agreements to repurchase	719,113	1,027,635
Long-term debt	333,371	365,421
Accrued interest payable	6,526	8,284
Derivative financial instruments	22,423	15,643
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash, interest-bearing bank deposits, and federal funds sold	734,680	1,622,366
Available for sale securities	2,208,789	4,496,900
Held to maturity securities	1,844,482
Loans, net	11,298,857	11,052,144
Loans held for sale	50,389	72,378
Accrued interest receivable	47,607	53,973
Derivative financial instruments	21,198	14,952
Deposits	14,772,951	15,713,579
Federal funds purchased	29,521	16,819
Securities sold under agreements to repurchase	719,113	1,027,635
Long-term debt	308,327	353,890
Accrued interest payable	6,526	8,284
Derivative financial instruments	$ 22,423	$ 15,643