ACQUISITION OF REVERE BANK - Fair values of acquired identifiable assets and liabilities assumed (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Apr. 01, 2020
Purchase price:
Fair value of common shares issued (12,768,949 shares) based on Sandy Spring's share price of $22.64			$ 289,089
Identifiable liabilities:
Goodwill			$ 500	$ 800
Revere Bank acquisition (in shares)	12,768,949		12,768,949
Revere Bank
Purchase price:
Fair value of common shares issued (12,768,949 shares) based on Sandy Spring's share price of $22.64	$ 289,089
Fair value of Revere stock options converted to Sandy Spring stock options	3,611
Cash paid for cashed-out Revere stock options	291
Cash for fractional shares	11
Total purchase price	293,002	$ 293,000
Identifiable assets:
Cash and cash equivalents	80,744
Investments available-for-sale	180,752
Loans	2,502,244
Premises and equipment	3,443
Accrued interest receivable	7,651
Core deposit intangible asset	18,360
Other assets	53,162
Total identifiable assets	2,846,356
Identifiable liabilities:
Deposits	2,322,422
Borrowings	205,514
Other liabilities	25,933
Total identifiable liabilities	2,553,869
Fair value of net assets acquired including identifiable intangible assets	292,487
Goodwill	$ 515
Closing price (in dollars per share)	$ 22.64			$ 22.64