Loans and Allowance for Loan Losses - Composition of Loans, Net of Unearned Income, Broken Out by Portfolio Segment and Class of Financing Receivable (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Unearned income on loans	$ 116,000	$ 115,000
Loans, net of unearned income	18,833,778	16,742,070
Commercial and Commercial Real Estate Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	16,845,582	14,970,747
Consumer Portfolio Segment
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	1,988,196	1,771,323
Consumer Portfolio Segment | Other
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	239,777	226,883
Consumer Portfolio Segment | Real estate secured loans
Financing Receivable, Recorded Investment [Line Items]
Loans, net of unearned income	$ 1,748,419	$ 1,544,440