BORROWINGS - Schedule of Subordinated Borrowing (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Oct. 05, 2020	Apr. 01, 2020
Subordinated Borrowing [Line Items]
Long-term borrowings	$ 172,639	$ 227,088
Subordinated Debt
Subordinated Borrowing [Line Items]
Subordinated debt, gross	175,000	231,000
Less: Debt held as investments by Sandy Spring	0	(3,000)
Add: Purchase accounting premium	0	1,669
Less: Debt issuance costs	(2,361)	(2,581)
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	$ 0	25,000
Add: Purchase accounting premium	2,200
Long-term borrowings	25,000
Interest rate			6.00%
Subordinated Debt | Revere acquisition
Subordinated Borrowing [Line Items]
Subordinated debt, gross	$ 0	$ 31,000
Add: Purchase accounting premium				$ 200
Long-term borrowings				$ 31,000
Interest rate				5.625%