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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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