Impairment of Investment Securities - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Other than temporary impairment charges recognized	$ 0	$ 0
Amortized Cost	880,416,000		$ 896,495,000
Securities available for sale, at fair value	908,236,000		902,292,000
Investment Securities, Continuous Unrealized Loss Position, Accumulated Loss	32,000		3,587,000
Federal Home Loan Bank Stock	11,300,000		15,100,000
Other Investments and Securities, at Cost	$ 1,700,000		1,700,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total	10
Corporate Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Securities available for sale, at fair value	$ 24,440,000		23,962,000
Equity Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Other than temporary impairment charges recognized	$ 0	$ 0
US Government-sponsored Enterprises Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Percent of Unrealized Losses	78.00%
Corporate Debt Securities [Member]
Debt and Equity Securities, FV-NI [Line Items]
Amortized Cost	$ 22,935,000		22,919,000
Securities available for sale, at fair value	$ 24,440,000		$ 23,962,000
Investment Securities in Unrealized Loss Position, Qualitative Disclosure, Number of Positions, Total	0		0