BORROWINGS - Schedule of Subordinated Borrowing (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Subordinated Borrowing [Line Items]
Subordinated debt	$ 230,300	$ 209,406
Subordinated Debt
Subordinated Borrowing [Line Items]
Subordinated debt, gross	231,000	200,000
Subordinated debt	230,300	199,009
Purchase accounting premium	1,954	1,894
Debt Issuance Costs, Gross	$ (2,654)	(2,885)
Interest rate	4.25%
Subordinated Debt | Fixed Floating
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 175,000	175,000
Subordinated Debt | WashingtonFirst Bankshares Inc
Subordinated Borrowing [Line Items]
Subordinated debt, gross	25,000	25,000
Subordinated debt	25,000
Purchase accounting premium	$ 2,200
Interest rate	6.00%
Subordinated Debt | Revere acquisition
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 31,000	0
Subordinated debt	31,000
Purchase accounting premium	$ 200
Interest rate	5.625%
Junior Subordinated Debt
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 0	10,310
Subordinated debt	0	10,397
Purchase accounting premium	0	$ 87
Junior Subordinated Debt | WashingtonFirst Bankshares Inc
Subordinated Borrowing [Line Items]
Subordinated debt	10,300
Purchase accounting premium	$ 100