BORROWINGS - Schedule of Subordinated Borrowing (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Oct. 05, 2020	Dec. 31, 2019
Subordinated Borrowing [Line Items]
Subordinated debt	$ 227,088		$ 209,406
Subordinated Debt
Subordinated Borrowing [Line Items]
Subordinated debt, gross	231,000		200,000
Less: Debt issuance costs	2,581		2,885
Subordinated debt	227,088		199,009
Add: Purchase accounting premium	1,669		1,894
Subordinated Debt | Fixed Floating
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 175,000		175,000
Interest rate	4.25%
Subordinated Debt | WashingtonFirst Bankshares Inc
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 25,000		25,000
Subordinated debt	25,000
Add: Purchase accounting premium	$ 2,200
Interest rate	4.81%	6.00%
Subordinated Debt | Revere acquisition
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 31,000		0
Subordinated debt	31,000
Add: Purchase accounting premium	$ 200
Interest rate	5.625%
Subordinated Debt | WashingtonFirst Bankshares Inc And Revere
Subordinated Borrowing [Line Items]
Less: WashingtonFirst and Revere sub debt held as investments by Sandy Spring	$ (3,000)		0
Junior Subordinated Debt
Subordinated Borrowing [Line Items]
Subordinated debt, gross	0		10,310
Subordinated debt	0		10,397
Add: Purchase accounting premium	0		$ 87
Junior Subordinated Debt | WashingtonFirst Bankshares Inc
Subordinated Borrowing [Line Items]
Subordinated debt	10,300
Add: Purchase accounting premium	$ 100