Fair Value Measurements - Estimated Fair Values of Financial Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 5,300,453	$ 4,556,125	$ 4,320,061	$ 3,524,979
Securities held to maturity	2,942,012	3,139,748
Loans, net	10,648,472	9,423,262
Deposits	23,490,603	20,688,786
Federal funds purchased and repurchase agreements	580,965	668,253
Junior subordinated deferrable interest debentures	137,115	123,712
Subordinated notes payable and other borrowings	100,000	100,000
Carrying Amount [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5,300,453	4,556,125
Securities held to maturity	2,942,012	3,139,748
Cash surrender value of life insurance policies	171,142	141,108
Accrued interest receivable	75,953	99,281
Deposits	23,490,603	20,688,786
Federal funds purchased and repurchase agreements	580,965	668,253
Junior subordinated deferrable interest debentures	137,115	123,712
Subordinated notes payable and other borrowings	100,000	100,000
Accrued interest payable	1,485	1,300
Carrying Amount [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	10,648,472	9,423,262
Estimated Fair Value [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	5,300,453	4,556,125
Securities held to maturity	2,970,785	3,029,663
Cash surrender value of life insurance policies	171,142	141,108
Accrued interest receivable	75,953	99,281
Deposits	23,491,181	20,689,323
Federal funds purchased and repurchase agreements	580,965	668,253
Junior subordinated deferrable interest debentures	137,115	123,712
Subordinated notes payable and other borrowings	94,622	92,552
Accrued interest payable	1,485	1,300
Estimated Fair Value [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 10,712,497	$ 9,582,734