LOANS HELD FOR INVESTMENT - Loan Portfolio Held for Investment (Parenthetical) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Financing Receivable, before Allowance for Credit Loss	$ 11,777,289	$ 9,002,205
Unaccreted fair value net purchase discounts	48,000	15,100
Commercial Portfolio Segment [Member] | COVID-19 [Member] | US Government Insured Loan Program [Member]
Financing Receivable, before Allowance for Credit Loss	406,000
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Financing Receivable, before Allowance for Credit Loss	3,202,590	2,230,876
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | Commercial loans secured by real estate [Member]
Financing Receivable, before Allowance for Credit Loss	$ 1,000,000	$ 719,000