Summary of Significant Accounting Policies - Additional Information (Detail)		12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2020 USD ($) office trust segment county	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Of 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		$ 1,731,000
Number of business segments (in segments) | segment		1
Marketable securities recorded		$ 3,025,000	$ 2,960,000
Total shareholders' equity		925,114,000	906,570,000	$ 827,373,000	$ 505,808,000
Gain (loss) on marketable equity securities		64,000	86,000	(64,000)
Security credit losses recognized		0	0	0
Credit loss		0	0	0
Retained Earnings
Significant Of Accounting Policies [Line Items]
Total shareholders' equity		381,999,000	367,794,000	$ 303,490,000	255,200,000
Cumulative Effect, Period of Adoption, Adjustment
Significant Of Accounting Policies [Line Items]
Total shareholders' equity			(12,983,000)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings
Significant Of Accounting Policies [Line Items]
Total shareholders' equity			$ (12,983,000)
Cumulative Effect, Period of Adoption, Adjustment | Adoption ASU 2016-01
Significant Of Accounting Policies [Line Items]
Total shareholders' equity					0
Decrease in deferred taxes	$ (18,000)
Cumulative Effect, Period of Adoption, Adjustment | Adoption ASU 2016-01 | Retained Earnings
Significant Of Accounting Policies [Line Items]
Total shareholders' equity					$ (62,000)
Held-to-maturity securities
Significant Of Accounting Policies [Line Items]
Interest Receivable		$ 735,000
California
Significant Of Accounting Policies [Line Items]
Number of counties (in counties) | county		29
Minimum
Significant Of Accounting Policies [Line Items]
Loans held-for-sale, term		5 years
Regulatory liability, amortization period		15 years
Maximum
Significant Of Accounting Policies [Line Items]
Loans held-for-sale, term		10 years
Regulatory liability, amortization period		30 years
North Valley Bancorp
Significant Of Accounting Policies [Line Items]
Number of subsidiary business trusts (in trusts) | trust		3
Furniture and equipment | Minimum
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		3 years
Furniture and equipment | Maximum
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		10 years
Land improvements and buildings | Minimum
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		15 years
Land improvements and buildings | Maximum
Significant Of Accounting Policies [Line Items]
Estimated useful lives of the related assets or lease terms		40 years