Mortgage Servicing Rights - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Risks Inherent in Servicing Assets and Servicing Liabilities [Line Items]
Unpaid principal balance of mortgage loans serviced for others	$ 346.1	$ 336.7
Minimum [Member]
Risks Inherent in Servicing Assets and Servicing Liabilities [Line Items]
Discount rates used in valuation model	10.00%
Prepayment speeds used in valuation model	20.90%
Maximum [Member]
Risks Inherent in Servicing Assets and Servicing Liabilities [Line Items]
Discount rates used in valuation model	13.50%
Prepayment speeds used in valuation model	46.90%