Composition of Loan Portfolio (Parenthetical) (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for Investment		$ 39,418,757	$ 38,359,022
Commercial and Industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for Investment	[1]	2,170,439	2,040,574
Commercial and Industrial | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for Investment		1,535,618	1,377,964
Lease financing, unearned income
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unearned income		59,088	65,041
Loans Held for Investment		634,821	662,610
Specialty Finance Loans and Leases [Member] | Commercial and Industrial | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for Investment		1,700,000	1,700,000
Other Commercial and Industrial Loans | Commercial and Industrial | Other loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for Investment		$ 491,700	$ 500,800

[1] Includes lease financing receivables, all of which were classified as Pass.