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Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2021 USD ($) office trust segment county	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 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	$ 1,830,000
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 loss	85,941,000	91,847,000	57,911,000		$ 30,616,000
Allowance for Credit Losses	$ 0	$ 0	$ 0
North Valley Bancorp
Significant Accounting Policies [Line Items]
Number of loan production offices (in offices) | office	3
Held-to-maturity securities
Significant Accounting Policies [Line Items]
Interest receivable	$ 668,000
ASU 2016-13
Significant Accounting Policies [Line Items]
Allowance for credit loss				$ 18,913,000
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	29

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Nonaccrual status loans contractual past due.

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