Commitments and Contingencies (Financial Instruments with Off-Balance Sheet Risk) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Loan commitments
Unadvanced portion of loans and leases	$ 808,681	$ 707,997
Unused lines of credit:
Home equity	528,251	487,476
Other consumer	25,374	50,404
Other commercial	380	347
Unused letters of credit:
Financial standby letters of credit	10,166	11,491
Performance standby letters of credit	4,652	3,075
Commercial and similar letters of credit	3,823	4,573
Commercial real estate
Loan commitments
Commitments to originate loans and leases	50,034	76,642
Commercial
Loan commitments
Commitments to originate loans and leases	78,058	75,713
Residential mortgage
Loan commitments
Commitments to originate loans and leases	25,998	16,363
Receive fixed, pay variable
Unused letters of credit:
Derivatives	1,101,193	714,500
Pay fixed, receive variable
Unused letters of credit:
Derivatives	1,101,193	714,500
Risk participation-out agreements
Unused letters of credit:
Derivatives	235,693	100,531
Risk participation-in agreements
Unused letters of credit:
Derivatives	55,281	35,838
Foreign exchange contracts | Buys foreign currency, sells U.S. currency
Unused letters of credit:
Derivatives	1,125	6,573
Foreign exchange contracts | Sells foreign currency, buys U.S. currency
Unused letters of credit:
Derivatives	$ 1,230	$ 6,582