Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 4,320,061	$ 3,524,979	$ 2,752,176	$ 2,907,592
Securities held to maturity	3,156,146	2,956,381
Loans, net	9,213,307	9,119,395
Deposits	19,978,894	19,497,366
Federal funds purchased and repurchase agreements	587,137	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	4,320,061	3,524,979
Securities held to maturity	3,156,146	2,956,381
Cash surrender value of life insurance policies	140,404	138,005
Accrued interest receivable	60,964	82,529
Deposits	19,978,894	19,497,366
Federal funds purchased and repurchase agreements	587,137	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	100,000	100,007
Accrued interest payable	1,407	1,804
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	9,213,307	9,119,395
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	4,320,061	3,524,979
Securities held to maturity	3,052,912	2,996,944
Cash surrender value of life insurance policies	140,404	138,005
Accrued interest receivable	60,964	82,529
Deposits	19,979,405	19,498,518
Federal funds purchased and repurchase agreements	587,137	561,061
Junior subordinated deferrable interest debentures	123,712	123,712
Subordinated notes payable and other borrowings	92,267	89,596
Accrued interest payable	1,407	1,804
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 9,313,193	$ 9,212,159