Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 2,901,355	$ 3,524,979	$ 2,295,251	$ 2,907,592
Securities held to maturity	3,166,692	2,956,381
Loans, net	9,139,143	9,119,395
Deposits	19,078,219	19,497,366
Federal funds purchased and repurchase agreements	549,823	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	100,000	100,007
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,901,355	3,524,979
Securities held to maturity	3,166,692	2,956,381
Cash surrender value of life insurance policies	139,704	138,005
Accrued interest receivable	95,334	82,529
Deposits	19,078,219	19,497,366
Federal funds purchased and repurchase agreements	549,823	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	100,000	100,007
Accrued interest payable	1,551	1,804
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	9,139,143	9,119,395
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	2,901,355	3,524,979
Securities held to maturity	3,070,349	2,996,944
Cash surrender value of life insurance policies	139,704	138,005
Accrued interest receivable	95,334	82,529
Deposits	19,078,952	19,498,518
Federal funds purchased and repurchase agreements	549,823	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	90,185	89,596
Accrued interest payable	1,551	1,804
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 9,262,790	$ 9,212,159