CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - UNAUDITED - USD ($) $ in Thousands	Total	Revision of Prior Period, Accounting Standards Update, Adjustment	Common Stock	Additional Paid-In Capital	Retained Earnings	Retained Earnings Revision of Prior Period, Accounting Standards Update, Adjustment	Accumulated Other Comprehensive Income/ (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASC 326 - Financial Instruments - Credit Losses		$ (2,223)				$ (2,223)
Balance at beginning of period at Dec. 31, 2019	$ 1,132,974		$ 34,970	$ 586,622	$ 515,714		$ (4,332)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(4,351)				(4,351)
Other comprehensive income, net of tax	19,156						19,156
Total other comprehensive income	14,805
Common stock dividends	(24,748)				(24,748)
Stock compensation expense	1,792			1,792
Fair value of common shares issued (12,768,949 shares) based on Sandy Spring's share price of $22.64	289,089		12,769	276,320
Conversion of Revere stock options	3,611			3,611
Stock option plan	147		9	138
Employee stock purchase plan	785		29	756
Restricted stock, net	(437)		44	(481)
Common stock repurchase	(25,702)		(820)	(24,882)
Balance at end of period at Jun. 30, 2020	$ 1,390,093		47,001	843,876	484,392		14,824
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance at beginning of period at Mar. 31, 2020	$ 1,116,334		34,165	562,891	512,934		6,344
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(14,338)				(14,338)
Other comprehensive income, net of tax	8,480						8,480
Total other comprehensive income	(5,858)
Common stock dividends	(14,204)				(14,204)
Stock compensation expense	1,038			1,038
Fair value of common shares issued (12,768,949 shares) based on Sandy Spring's share price of $22.64	289,089		12,769	276,320
Conversion of Revere stock options	3,611			3,611
Stock option plan	25		3	22
Employee stock purchase plan	495		20	475
Restricted stock, net	(437)		44	(481)
Balance at end of period at Jun. 30, 2020	1,390,093		47,001	843,876	484,392		14,824
Balance at beginning of period at Dec. 31, 2020	1,469,955		47,057	846,922	557,271		18,705
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	132,727				132,727
Other comprehensive income, net of tax	(13,871)						(13,871)
Total other comprehensive income	118,856
Common stock dividends	(30,420)				(30,420)
Stock compensation expense	2,309			2,309
Stock option plan	2,016		140	1,876
Employee stock purchase plan	1,102		38	1,064
Restricted stock, net	(1,538)		78	(1,616)
Balance at end of period at Jun. 30, 2021	1,562,280		47,313	850,555	659,578		4,834
Balance at beginning of period at Mar. 31, 2021	1,511,694		47,187	849,606	617,553		(2,652)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	57,263				57,263
Other comprehensive income, net of tax	7,486						7,486
Total other comprehensive income	64,749
Common stock dividends	(15,238)				(15,238)
Stock compensation expense	1,362			1,362
Stock option plan	485		38	447
Employee stock purchase plan	561		18	543
Restricted stock, net	(1,333)		70	(1,403)
Balance at end of period at Jun. 30, 2021	$ 1,562,280		$ 47,313	$ 850,555	$ 659,578		$ 4,834