Fair Value - Schedule of Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	$ 1,458,120	$ 2,048,442
Held to maturity securities	2,653,867	1,710,465
Loans, net	11,596,249	11,441,631
Loans held for sale	18,444	50,605
Derivative financial instruments	16,062	20,093
Deposits	15,054,871	15,744,188
Long-term debt	376,664	396,589
Derivative financial instruments	15,929	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	888,005	1,948,679
Available for sale securities	9,470	25,794
Held to maturity securities	100,688
Accrued interest receivable	42,039	45,616
Federal funds purchased	22,193	25,704
Securities sold under agreements to repurchase	760,586	613,429
Accrued interest payable	5,694	4,814
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	1,448,650	2,022,648
Held to maturity securities	2,553,179	1,710,465
Loans, net	27,981	72,694
Loans held for sale	18,444	50,605
Derivative financial instruments	16,062	20,093
Long-term debt	384,239	410,791
Derivative financial instruments	15,929	21,100
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, net	11,528,860	11,494,409
Deposits	15,017,314	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	888,005	1,948,679
Available for sale securities	1,458,120	2,048,442
Held to maturity securities	2,653,867	1,710,465
Loans, net	11,556,841	11,567,103
Loans held for sale	18,444	50,605
Accrued interest receivable	42,039	45,616
Derivative financial instruments	16,062	20,093
Deposits	15,017,314	15,757,044
Federal funds purchased	22,193	25,704
Securities sold under agreements to repurchase	760,586	613,429
Long-term debt	384,239	410,791
Accrued interest payable	5,694	4,814
Derivative financial instruments	15,929	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	888,005	1,948,679
Available for sale securities	1,458,120	2,048,442
Held to maturity securities	2,666,082	1,668,018
Loans, net	11,596,250	11,441,631
Loans held for sale	18,444	50,605
Accrued interest receivable	42,039	45,616
Derivative financial instruments	16,062	20,093
Deposits	15,054,871	15,744,188
Federal funds purchased	22,193	25,704
Securities sold under agreements to repurchase	760,586	613,429
Long-term debt	376,664	396,589
Accrued interest payable	5,694	4,814
Derivative financial instruments	$ 15,929	$ 21,100