Loans - Loan Categories Narrative (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Gross loans
Outstanding loan balances to related parties		$ 48,700	$ 35,100
Unadvanced lines of credit available to related parties		23,200	32,100
New loans and net increases to loan balances to related parties during period		16,100	23,800
Principal paydowns on related party loans		$ 11,400	3,700
Number of paycheck protection loans | loan		2,633
Gross Loans		$ 3,087,246	2,568,562
Provision for loan losses	$ 2,250	12,499	1,180	$ 2,250
Commercial
Gross loans
Participation loans amount		77,100	104,300
Gross Loans		2,740,288	2,212,883
Participations loans sold that are still serviced amount		65,300	80,200
Tax exempt interest income on qualified commercial loans		2,000	2,100	$ 2,300
Average tax exempt loan balances		58,000	59,200
Residential mortgages
Gross loans
Gross Loans		252,971	247,373
Provision for loan losses		335	35
Amount of loans serviced for others		$ 13,700	15,700
SBA paycheck protection program
Gross loans
Loans receivable, interest rate		1.00%
Loans and leases receivable term, minimum		2 years
Loans and leases receivable term, maximum		5 years
Gross Loans		$ 453,084	$ 0
Gross deferred Income		17,200
Provision for loan losses		10,000
Amortization of deferred loan origination fees, net		7,200
SBA paycheck protection program | Average
Gross loans
Gross Loans		$ 180