Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan placed on nonaccrual when past due, number of days	90 days
Loan participations	$ 868,100,000
Loan participations sold	405,200,000
Loan participations retained	$ 462,900,000
Troubled debt restructuring term	6 months
Minimum number of days for loan charge off to be recorded	120 days
Maximum number of days for loan charge off to be recorded	180 days
Nonaccrual period for loans	90 days
Financing receivable TDR's included with non-accrual loans	$ 13,800,000	$ 26,300,000
Financing receivable troubled debt restructurings specific reserves	900,000	3,000,000.0
Unfunded commitments on TDRs	2,300,000	4,400,000
Increase (decrease) in allowance for loan losses	2,000,000.0	0	$ 2,700,000
Allowance for loan losses charge-offs	$ 3,900,000	0	$ 200,000
Number of days for a loan to be considered to be in payment default	90 days
Outstanding loans including principal, interest, fees and penalties	$ 223,300,000	246,900,000
Accretion of income	11,300,000	12,300,000
Financing receivable allowance for loan losses related to purchased loans	195,000	478,000
Maximum [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent	$ 250,000
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital	194.00%
Regulatory guideline limit	300.00%
Financing receivable allowance for loan losses related to purchased loans		$ 319,000
Credit Concentration Risk [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage	10.00%
Credit Concentration Risk [Member] | Non Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage	10.00%