Fair Value - Schedule of Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	$ 2,853,539	$ 2,048,442
Held to maturity securities	1,848,886	1,710,465
Loans, net	11,344,985	11,441,631
Loans held for sale	34,813	50,605
Derivative financial instruments	21,254	20,093
Deposits	15,253,351	15,744,188
Long-term debt	393,920	396,589
Derivative financial instruments	22,075	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	777,152	1,948,679
Available for sale securities	9,278	25,794
Accrued interest receivable	46,390	45,616
Federal funds purchased	32,814	25,704
Securities sold under agreements to repurchase	689,723	613,429
Accrued interest payable	6,374	4,814
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	2,844,261	2,022,648
Held to maturity securities	1,848,886	1,710,465
Loans, net	42,504	72,694
Loans held for sale	34,813	50,605
Derivative financial instruments	21,254	20,093
Long-term debt	406,304	410,791
Derivative financial instruments	22,075	21,100
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, net	11,325,804	11,494,409
Deposits	15,263,799	15,757,044
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	777,152	1,948,679
Available for sale securities	2,853,539	2,048,442
Held to maturity securities	1,848,886	1,710,465
Loans, net	11,368,308	11,567,103
Loans held for sale	34,813	50,605
Accrued interest receivable	46,390	45,616
Derivative financial instruments	21,254	20,093
Deposits	15,263,799	15,757,044
Federal funds purchased	32,814	25,704
Securities sold under agreements to repurchase	689,723	613,429
Long-term debt	406,304	410,791
Accrued interest payable	6,374	4,814
Derivative financial instruments	22,075	21,100
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	777,152	1,948,679
Available for sale securities	2,853,539	2,048,442
Held to maturity securities	1,808,740	1,668,018
Loans, net	11,344,985	11,441,631
Loans held for sale	34,813	50,605
Accrued interest receivable	46,390	45,616
Derivative financial instruments	21,254	20,093
Deposits	15,253,351	15,744,188
Federal funds purchased	32,814	25,704
Securities sold under agreements to repurchase	689,723	613,429
Long-term debt	393,920	396,589
Accrued interest payable	6,374	4,814
Derivative financial instruments	$ 22,075	$ 21,100