Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 3,218,460	$ 3,524,979	$ 2,128,734
Securities held to maturity	3,068,008	2,956,381	365,603
Loans, net	9,068,762	9,119,395	8,019,532
Deposits	19,043,904	19,497,366	16,909,021
Federal funds purchased and repurchase agreements	510,102	561,061	637,098
Junior subordinated deferrable interest debentures	123,712	123,712	123,712
Subordinated notes payable and other borrowings	100,002	100,007	100,022
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,218,460	3,524,979
Securities held to maturity	3,068,008	2,956,381
Cash surrender value of life insurance policies	138,898	138,005
Accrued interest receivable	66,779	82,529
Deposits	19,043,904	19,497,366
Federal funds purchased and repurchase agreements	510,102	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	100,002	100,007
Accrued interest payable	1,697	1,804
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	9,068,762	9,119,395
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,218,460	3,524,979
Securities held to maturity	3,079,426	2,996,944
Cash surrender value of life insurance policies	138,898	138,005
Accrued interest receivable	66,779	82,529
Deposits	19,044,847	19,498,518
Federal funds purchased and repurchase agreements	510,102	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	90,706	89,596
Accrued interest payable	1,697	1,804
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 9,157,877	$ 9,212,159