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Summary of Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 9 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) business_trust | Sep. 30, 2020 USD ($) business_trust segment county | Jun. 30, 2020 USD ($) | Jan. 01, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Jun. 30, 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 | $ 1,727,699 | $ 1,727,699 |  |  |  |  |  |
Number of business segments (in segments) | segment |  | 1 |  |  |  |  |  |
Loans contractual past due |  | 90 days |  |  |  |  |  |
Allowance for credit loss | 87,575,000 | $ 87,575,000 | $ 79,739,000 |  | $ 30,616,000 | $ 31,537,000 | $ 32,868,000 | $ 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 | $ 795,000 | $ 795,000 |  |  |  |  |  |
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 |  |  |  |  |  |

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