LOANS HELD FOR INVESTMENT - Loan Portfolio Held for Investment (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, before Allowance for Credit Loss	$ 11,847,275	$ 9,002,205
Unaccreted fair value net purchase discounts	51,800	15,100
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member]
Financing Receivable, before Allowance for Credit Loss	3,226,843	2,230,876
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | COVID-19 | Small Business Administration
Financing Receivable, before Allowance for Credit Loss	453,400
Commercial Portfolio Segment [Member] | Commercial And Industrial Sector [Member] | Commercial loans secured by real estate
Financing Receivable, before Allowance for Credit Loss	$ 1,000	$ 719,000