Fair Values of Assets and Liabilities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Random sample percentage of securities for each quarter	100.00%
Total gains or losses included in earnings	$ 0	$ (198,000)
Interest rate swaps transferred into Level 3 from Level 2	0	0
Corporate securities transferred from Level 3 to Level 2	0	0
Impaired loans considered to be credit risk of non-collection	250,000
Updated appraisal requirement floor	250,000
Other real estate owned	1,215,000	2,228,000
Financial instrument of credit risk	104,000	138,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings On Assets Held At Year End	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Between Levels 1 And 2	0	0
Provision for credit losses	53,472,000
Securities available for sale	831,223,000	902,292,000
Other Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains or losses included in earnings	0	(198,000)
Interest rate swaps transferred into Level 3 from Level 2	0
Corporate securities transferred from Level 3 to Level 2	$ 0	$ 0