Loans and the Allowance for Credit Losses - Additional Information (Detail)	3 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 Contract	Dec. 31, 2022 USD ($)
Financing Receivable, Recorded Investment [Line Items]
Net deferred loan fee income (costs)	$ (11,300,000)		$ (9,600,000)
Un-accreted discount on purchased loans from acquisitions	14,500,000		18,000,000
Reserve on accrued interest related to CARES Act	$ 100,000
Percentage of national unemployment projection	4.30%
Expected average percentage of national unemployment in future	4.70%
Accrued interest receivable for loans	$ 57,700,000		51,800,000
Accrued interest receivable for loans related to CARES Act	15,800,000		17,000,000
Aggregate amount of residential real estate, home equity and consumer loans classified as substandard	20,600,000		24,800,000
Internally assigned loan grades to residential real estate, home equity and consumer loans	4,200,000		5,900,000
Unfunded commercial loan commitments	13,800,000		25,000,000
TDRs defaulted | Contract		0
Other real estate owned	1,231,000		1,397,000
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Other real estate owned			100,000
Foreclosure proceedings in process on residential real estate loans	4,400,000		$ 4,900,000
90 Days or More Past Due [Member]
Financing Receivable, Recorded Investment [Line Items]
Modifications loans	0
Commercial and Industrial [Member] | Maximum [Member]
Financing Receivable, Recorded Investment [Line Items]
Annual sales of borrowers	100,000,000
Unfunded Loan Commitment [Member]
Financing Receivable, Recorded Investment [Line Items]
Modifications loans	$ 3,200,000