ACQUISITION OF WASHINGTONFIRST BANKSHARES INC -acquired assets liabilities (Details) - USD ($) $ in Thousands		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Dec. 31, 2017
Purchase price:
Fair value of common share issue (11,446,197 shares), based on Sandy Spring's share price of $36.02	$ 446,640	$ 446,640
Cash for fractional shares	10
Total purchase price	446,650
Identifiable assets and liabilities:
Cash and cash equivalents	32,497
Residential mortgage loans held for sale	25,789
Investment securities	302,321
Loans	1,680,278
Premises and equipment	4,602
Other Real Estate Owned	497
Accrued Interest Receivable	6,648
Other Intangible assets	11,370
Other Assets	34,654
Total identifiable assets	2,098,656		$ 2,100,000
Deposits	1,610,327
Other Liabilities	283,808
Borrowings	17,525
Total identifiable liabilities	1,911,660
Provisional fair value of net assets acquired including identifiable intangible assets	186,996
Provisional resulting goodwill	$ 259,654