Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
INTEREST INCOME
Loans, taxable	$ 54,720	$ 46,615	$ 39,103
Loans, non-taxable	33,167	31,936	26,910
Securities available-for-sale, taxable	7,125	6,748	4,987
Securities available-for-sale, non-taxable	1,087	1,587	1,119
Federal Home Loan Bank of Boston dividends	953	1,116	872
Securities held-to-maturity	58,036	45,556	38,348
Federal funds sold, interest-bearing deposits in other banks and short-term investments	4,051	3,498	2,097
Total interest income	159,139	137,056	113,436
INTEREST EXPENSE
Savings and NOW deposits	21,183	11,757	6,296
Money market accounts	21,170	13,922	5,626
Time deposits	11,804	10,208	7,919
Securities sold under agreements to repurchase	2,347	976	496
Other borrowed funds and subordinated debentures	6,846	7,617	7,483
Total interest expense	63,350	44,480	27,820
Net interest income	95,789	92,576	85,616
Provision for loan losses (Note 6)	1,250	1,350	1,790
Net interest income after provision for loan losses	94,539	91,226	83,826
OTHER OPERATING INCOME
Service charges on deposit accounts	9,220	8,560	8,586
Lockbox fees	3,973	3,274	3,290
Brokerage commissions	277	348	353
Net gains on sales of securities	61	302	47
Gains on sales of mortgage loans	412		370
Other income	4,456	3,764	3,906
Total other operating income	18,399	16,248	16,552
OPERATING EXPENSES
Salaries and employee benefits (Note 17)	44,014	42,710	40,517
Occupancy	6,246	6,092	6,140
Equipment	3,238	3,132	2,892
FDIC assessments	729	1,471	1,581
Other (Note 20)	17,902	16,288	15,989
Total operating expenses	72,129	69,693	67,119
Income before income taxes	40,809	37,781	33,259
Provision for income taxes (Note 16)	1,110	1,568	10,958
Net income	$ 39,699	$ 36,213	$ 22,301
Class A Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	3,633,044	3,608,179	3,604,029
Weighted average number of shares outstanding, diluted	5,567,909	5,567,909	5,567,909
Basic earnings per share	$ 8.63	$ 7.89	$ 4.86
Diluted earnings per share	$ 7.13	$ 6.50	$ 4.01
Class B Common Stock [Member]
SHARE DATA (Note 14)
Weighted average number of shares outstanding, basic	1,934,865	1,959,730	1,963,880
Weighted average number of shares outstanding, diluted	1,934,865	1,959,730	1,963,880
Basic earnings per share	$ 4.31	$ 3.95	$ 2.43
Diluted earnings per share	$ 4.31	$ 3.95	$ 2.43