Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) community Office	Dec. 31, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of banking offices (in offices) | Office	40
Number of retirement communities with banking and trust services (in communities) | community	14
Accrued interest receivable	$ 15,300,000
Federal Reserve Bank stock	14,600,000	$ 14,600,000
FHLB Stock	13,500,000	13,500,000
Other-than-temporary impairment of equity securities	$ 0	$ 0
Number of days loan or lease past due for nonaccrual of interest status	90 days
Operating lease, options to extend	Most leases include one or more options to renew, with renewal terms generally containing one or more five-year renewal options.
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, remaining lease term	4 months
Minimum [Member] | Customer Related [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangibles and other identified intangibles with finite useful lives	5 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, remaining lease term	22 years
Period over which optioned shares become exercisable	10 years
Maximum [Member] | Core Deposits and Other Intangible Assets [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangibles and other identified intangibles with finite useful lives	15 years
Maximum [Member] | Customer Related [Member]
Summary Of Significant Accounting Policies [Line Items]
Intangibles and other identified intangibles with finite useful lives	12 years
Building [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	40 years
Owned Building on Leased Land or Land Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment, description	new buildings constructed on leased land or land improvements, the estimated useful life is the initial term including anticipated renewable terms, typically not exceeding twenty-five years
Furniture, Fixtures and Equipment [Member] | Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Furniture, Fixtures and Equipment [Member] | Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	10 years
Restricted Stock [Member]
Summary Of Significant Accounting Policies [Line Items]
Period over which optioned shares become exercisable	3 years
Stock Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Period over which optioned shares become exercisable	3 years
Held-to-maturity Securities [Member]
Summary Of Significant Accounting Policies [Line Items]
Accrued interest receivable	$ 441,000
Available-for-sale Securities [Member]
Summary Of Significant Accounting Policies [Line Items]
Accrued interest receivable	$ 477,000