Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	6 Months Ended
	Jun. 30, 2020 USD ($) business_trust	Jun. 30, 2020 USD ($) business_trust county segment	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Number of subsidiary business trusts (in trusts) | business_trust		5
Number of loan production offices (in offices) | business_trust	2	2
Company's investments in the trusts	$ 1,761,000	$ 1,761,000
Number of business segments (in segments) | segment		1
Loans contractual past due		90 days
Allowance for credit loss	79,739,000	$ 79,739,000	$ 57,911,000		$ 30,616,000	$ 32,868,000	$ 32,064,000	$ 32,582,000
Goodwill impairment loss	$ 0	$ 0
North Valley Bancorp
Significant Accounting Policies [Line Items]
Number of loan production offices (in offices) | business_trust	3	3
Held-to-maturity securities
Significant Accounting Policies [Line Items]
Interest receivable	$ 860,000	$ 860,000
Financial asset acquired with credit deterioration
Significant Accounting Policies [Line Items]
Allowance for credit loss				$ 481,000
ASU 2016-13
Significant Accounting Policies [Line Items]
Allowance for credit loss				18,913,000
ASU 2016-13 | Retained Earnings
Significant Accounting Policies [Line Items]
Taxes				5,449,000
Cumulative effective from change in accounting policies				$ 12,983,000
Minimum
Significant Accounting Policies [Line Items]
Loan term	5 years	5 years
Amortization period		15 years
Maximum
Significant Accounting Policies [Line Items]
Loan term	10 years	10 years
Amortization period		30 years
California
Significant Accounting Policies [Line Items]
Number of counties (in counties) | county		29