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Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 01, 2020 | Jun. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Increase to retained earnings for cumulative effect of adopting ASU tax |  | $ 925,000 |  |
Addition to allowance for credit losses for financial assets purchased with credit deterioration |  |  | $ 502,000 |
Accrued interest receivable |  | 31,000,000.0 | $ 31,000,000.0 | $ 27,400,000
Paycheck Protection Program, CARES Act [Member] | Small Business Administration (SBA) [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Percentage of processing fees |  |  | 5.00% |
Processing fees received from the SBA |  |  | $ 30,000,000.0 |
ASU No. 2016-13 [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Increase to retained earnings for cumulative effect of adopting ASU | $ 2,300,000 |  |  |
Increase to retained earnings for cumulative effect of adopting ASU tax | 925,000 |  |  |
Addition to allowance for credit losses for financial assets purchased with credit deterioration | $ 1,200,000 |  |  |
Accrued interest receivable |  | $ 31,000,000.0 | $ 31,000,000.0 |
Delinquency period placed on nonaccrual status loan |  |  | 90 days |
Commercial loans are charged off deemed uncollectible period |  | 180 days | 180 days |
ASU No. 2016-13 [Member] | U.S. Treasury or U.S. Government-sponsored Entities and Agencies [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Percentage of available for sale securities held |  |  | 95.00% | 95.00%

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