Fair Value of Financial Instruments - Schedule of Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	$ 2,048,442	$ 4,496,900
Held to maturity securities	1,710,465
Loans, net	11,441,631	11,052,145
Loans held for sale	50,605	72,378
Derivative financial instruments	20,093
Deposits	15,744,188	15,713,579
Long-term debt	396,589	353,890
Derivative financial instruments	21,100
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	1,948,679	1,622,366
Available for sale securities	25,794	260,814
Accrued interest receivable	45,616	53,973
Federal funds purchased	25,704	16,819
Securities sold under agreements to repurchase	613,429	1,027,635
Accrued interest payable	4,814	8,284
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	2,022,648	4,236,086
Held to maturity securities	1,710,465
Loans, net	76,413	55,252
Loans held for sale	50,605	72,378
Derivative financial instruments	20,093	14,952
Long-term debt	410,791	365,421
Derivative financial instruments	21,100	15,643
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, net	11,425,653	11,134,410
Deposits	15,757,044	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	1,948,679	1,622,366
Available for sale securities	2,048,442	4,496,900
Held to maturity securities	1,710,465
Loans, net	11,502,066	11,189,662
Loans held for sale	50,605	72,378
Accrued interest receivable	45,616	53,973
Derivative financial instruments	20,093	14,952
Deposits	15,757,044	15,737,667
Federal funds purchased	25,704	16,819
Securities sold under agreements to repurchase	613,429	1,027,635
Long-term debt	410,791	365,421
Accrued interest payable	4,814	8,284
Derivative financial instruments	21,100	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	1,948,679	1,622,366
Available for sale securities	2,048,442	4,496,900
Held to maturity securities	1,668,018
Loans, net	11,441,631	11,052,145
Loans held for sale	50,605	72,378
Accrued interest receivable	45,616	53,973
Derivative financial instruments	20,093	14,952
Deposits	15,744,188	15,713,579
Federal funds purchased	25,704	16,819
Securities sold under agreements to repurchase	613,429	1,027,635
Long-term debt	396,589	353,890
Accrued interest payable	4,814	8,284
Derivative financial instruments	$ 21,100	$ 15,643