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)
Balance at beginning of period at Dec. 31, 2019	$ 1,132,974	$ (2,221)	$ 34,970	$ 586,622	$ 515,714	$ (2,221)	$ (4,332)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,291				40,291
Other comprehensive loss, net of tax	21,825						21,825
Total other comprehensive income	62,116
Common stock dividends	(38,953)				(38,953)
Stock compensation expense	2,824			2,824
Revere Bank acquisition	289,089		12,769	276,320
Conversion of Revere stock options	3,611			3,611
Stock option plan	165		11	154
Employee stock purchase plan	1,309		50	1,259
Restricted stock, net	(463)		46	(509)
Common stock repurchase	(25,702)		(820)	(24,882)
Balance at end of period at Sep. 30, 2020	1,424,749		47,026	845,399	514,831		17,493
Balance at beginning of period at Jun. 30, 2020	1,390,093	$ 2	47,001	843,876	484,392	$ 2	14,824
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	44,642				44,642
Other comprehensive loss, net of tax	2,669						2,669
Total other comprehensive income	47,311
Common stock dividends	(14,205)				(14,205)
Stock compensation expense	1,032			1,032
Stock option plan	18		2	16
Employee stock purchase plan	524		21	503
Restricted stock, net	(26)		2	(28)
Balance at end of period at Sep. 30, 2020	1,424,749		47,026	845,399	514,831		17,493
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	189,703				189,703
Other comprehensive loss, net of tax	(19,831)						(19,831)
Total other comprehensive income	169,872
Common stock dividends	(45,673)				(45,673)
Stock compensation expense	3,640			3,640
Stock option plan	2,583		193	2,390
Employee stock purchase plan	1,581		49	1,532
Restricted stock, net	(1,577)		82	(1,659)
Common stock repurchase	(54,321)		(1,262)	(53,059)
Balance at end of period at Sep. 30, 2021	1,546,060		46,119	799,766	701,301		(1,126)
Balance at beginning of period at Jun. 30, 2021	1,562,280		47,313	850,555	659,578		4,834
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	56,976				56,976
Other comprehensive loss, net of tax	(5,960)						(5,960)
Total other comprehensive income	51,016
Common stock dividends	(15,253)				(15,253)
Stock compensation expense	1,331			1,331
Stock option plan	567		53	514
Employee stock purchase plan	479		11	468
Restricted stock, net	(39)		4	(43)
Common stock repurchase	(54,321)		(1,262)	(53,059)
Balance at end of period at Sep. 30, 2021	$ 1,546,060		$ 46,119	$ 799,766	$ 701,301		$ (1,126)