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, 2018	$ 1,067,903		$ 35,531	$ 606,573	$ 441,553		$ (15,754)
Net income	87,976				87,976
Other comprehensive income, net of tax	13,046						13,046
Total comprehensive income	101,022
Common stock dividends	(31,509)				(31,509)
Stock compensation expense	2,259			2,259
Stock option plan	259		12	247
Director's stock purchase plan	30		1	29
Employee stock purchase plan	779		27	752
Restricted stock, net	(702)		55	(757)
Balance at end of period at Sep. 30, 2019	1,140,041		35,626	609,103	498,020		(2,708)
Balance at beginning of period at Jun. 30, 2019	1,119,445		35,615	608,006	479,389		(3,565)
Net income	29,383				29,383
Other comprehensive income, net of tax	857						857
Total comprehensive income	30,240
Common stock dividends	(10,752)				(10,752)
Stock compensation expense	786			786
Stock option plan	35		1	34
Employee stock purchase plan	287		10	277
Balance at end of period at Sep. 30, 2019	1,140,041		35,626	609,103	498,020		(2,708)
Balance at beginning of period at Dec. 31, 2019	1,132,974		34,970	586,622	515,714		(4,332)
Net income	40,291				40,291		0
Other comprehensive income, net of tax	21,825						21,825
Total comprehensive income	62,116
Common stock dividends	(38,953)				(38,953)
Stock compensation expense	2,824			2,824	0
Revere Bank acquisition	289,089		12,769	276,320
Stock option plan	165		11	154
Conversion of Revere stock options	3,611		0	3,611
Employee stock purchase plan	1,309		50	1,259
Restricted stock, net	(463)		46	(509)
Adoption of ASC 326 - Financial Instruments - Credit Losses		$ (2,221)				$ (2,221)
Repurchase of common stock - 820,328 shares	(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		47,001	843,876	484,392		14,824
Net income	44,642				44,642
Other comprehensive income, net of tax	2,669						2,669
Total 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)
Adoption of ASC 326 - Financial Instruments - Credit Losses		$ 2				$ 2
Balance at end of period at Sep. 30, 2020	$ 1,424,749		$ 47,026	$ 845,399	$ 514,831		$ 17,493
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member