Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
INTEREST INCOME
Loans, taxable	$ 57,554	$ 54,720	$ 46,615
Loans, non-taxable	28,329	33,167	31,936
Securities available-for-sale, taxable	3,264	7,125	6,748
Securities available-for-sale, non-taxable	348	1,087	1,587
Federal Home Loan Bank of Boston dividends	626	953	1,116
Securities held-to-maturity	58,072	58,036	45,556
Federal funds sold, interest-bearing deposits in other banks and short-term investments	843	4,051	3,498
Total interest income	149,036	159,139	137,056
INTEREST EXPENSE
Savings and NOW deposits	8,880	21,183	11,757
Money market accounts	15,012	21,170	13,922
Time deposits	11,573	11,804	10,208
Securities sold under agreements to repurchase	1,376	2,347	976
Other borrowed funds and subordinated debentures	5,366	6,846	7,617
Total interest expense	42,207	63,350	44,480
Net interest income	106,829	95,789	92,576
Provision for loan losses (Note 6)	5,825	1,250	1,350
Net interest income after provision for loan losses	101,004	94,539	91,226
OTHER OPERATING INCOME
Service charges on deposit accounts	8,818	9,220	8,560
Lockbox fees	3,745	3,973	3,274
Brokerage commissions	271	277	348
Net gains on sales of securities	0	61	302
Gains on sales of mortgage loans		412
Other income	6,266	4,456	3,764
Total other operating income	19,100	18,399	16,248
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	45,303	44,014	42,710
Occupancy	6,040	6,246	6,092
Equipment	3,495	3,238	3,132
FDIC assessments	1,174	729	1,471
Other (Note 20)	17,476	17,902	16,288
Total operating expenses	73,488	72,129	69,693
Income before income taxes	46,616	40,809	37,781
Provision for income taxes (Note 16)	4,407	1,110	1,568
Net income	$ 42,209	$ 39,699	$ 36,213
Class A Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,653,939	3,633,044	3,608,179
Weighted average number of shares outstanding, diluted	5,567,909	5,567,909	5,567,909
Basic earnings per share	$ 9.15	$ 8.63	$ 7.89
Diluted earnings per share	$ 7.58	$ 7.13	$ 6.50
Class B Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,913,970	1,934,865	1,959,730
Weighted average number of shares outstanding, diluted	1,913,970	1,934,865	1,959,730
Basic earnings per share	$ 4.58	$ 4.31	$ 3.95
Diluted earnings per share	$ 4.58	$ 4.31	$ 3.95