Summary of Significant Accounting Policies - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2021 USD ($) trust office county segment	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)
Significant Accounting Policies [Line Items]
Number of subsidiary business trusts (in trusts) | trust	5
Number of loan production offices (in offices) | office	2
Company's investments in the trusts	$ 1,737,000
Number of business segment (in segments) | segment	1
Loans contractual past due	90 days
Held-to-maturity, allowance for credit loss	$ 0	$ 0	$ 0
Allowance for credit losses	$ 0	$ 0	$ 0
North Valley Bancorp
Significant Accounting Policies [Line Items]
Number of loan production offices (in offices) | office	3
Minimum
Significant Accounting Policies [Line Items]
Loan term	5 years
Amortization period	15 years
Maximum
Significant Accounting Policies [Line Items]
Loan term	10 years
Amortization period	30 years
California
Significant Accounting Policies [Line Items]
Number of counties (in counties) | county	28