Leases - Narrative (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) location lease	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Disclosure of Leases [Line Items]
Finance Lease, Liability, Statement of Financial Position [Extensible Enumeration]	Other liabilities	Other liabilities
Payment of lease payments	$ 15.6	$ 14.2
Rent expense under operating leases			$ 16.2
Number of leases terminated | lease	4
Operating lease, right-of-use asset, amortization expense	$ 1.7
Number of lease locations closed | location	3
Number of leases sublet | lease	5
Century Bancorp, Inc.
Disclosure of Leases [Line Items]
Number of leases terminated | lease	1
Number of leases sublet | lease	2
Equipment
Disclosure of Leases [Line Items]
Rent expense under operating leases			$ 0.7
Operating Lease Right-Of-Use Assets, Lease Abandonment
Disclosure of Leases [Line Items]
Impairment, long-lived asset, held-for-use	$ 0.3
Operating Lease Right-Of-Use Assets, Sublease
Disclosure of Leases [Line Items]
Impairment, long-lived asset, held-for-use	$ 0.5
Minimum
Disclosure of Leases [Line Items]
Operating lease remaining lease term	1 year
Maximum
Disclosure of Leases [Line Items]
Operating lease remaining lease term	25 years