LOAN PORTFOLIO - Loan Portfolio Held for Investment (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Gross	$ 8,996,816	$ 8,858,123	$ 8,695,890
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Gross	3,854,962	3,750,202
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Gross	2,211,731	2,148,111	$ 2,061,773
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | Commercial loans secured by real estate
Loans and Leases Receivable Disclosure [Line Items]
Loans and Leases Receivable, Gross	$ 782,200	$ 796,800