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Description of Business and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) |  | 12 Months Ended
--- | --- | ---
Jan. 01, 2020 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Days from origination after which mortgage loan are sold |  | 30 days |  |
Loan upon which a default of principal or interest has existed for a period |  | 90 days |  |
Intangible assets, Estimated useful lives |  | 7 years 2 months 12 days |  |
Advertising costs |  | $ 3,300,000 | $ 3,800,000 | $ 3,500,000
Revenue recognition, description of performance obligation |  | the Company offers various services to customers that generate revenue. Contract performance typically occurs in one year or less. |  |
Incremental cost of obtaining contract |  | true |  |
Revenue recognition, description of payment terms |  | Revenue is recognized when the transactions occur or as services are performed over primarily monthly or quarterly periods. Payment is typically received in the period the transactions occur, or in some cases, within 90 days of the service period. |  |
Increase to retained earnings for cumulative effect of adopting ASU |  | $ 871,161,000 | 815,488,000 |
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 |  | 502,000 |  |
Accrued interest receivable |  | $ 26,000,000.0 | $ 24,500,000 |
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 |  | $ 30,200,000 |  |
CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [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 |  |
ASU No. 2018-13 [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] | true |  |  |
Change in Accounting Principle, Accounting Standards Update, Adoption Date | Jan. 01, 2020 |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] | true |  |  |
ASC 326 [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] | true |  |  |
Change in Accounting Principle, Accounting Standards Update, Adoption Date | Jan. 01, 2020 |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] | true |  |  |
Addition to allowance for credit losses for financial assets purchased with credit deterioration | $ 1,200,000 |  |  |
Accrued interest receivable |  | $ 26,000,000.0 |  |
Delinquency period placed on nonaccrual status loan |  | 90 days |  |
Commercial loans are charged off deemed uncollectible period |  | 180 days |  |
ASC 326 [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% |
ASC 326 [Member] | CUMULATIVE EFFECT, PERIOD OF ADOPTION, ADJUSTMENT [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 |  |  |
Core Deposit Intangibles [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Intangible assets, Estimated useful lives |  | 7 years 3 months 18 days |  |
Core Deposit Intangibles [Member] | Minimum [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Intangible assets, Estimated useful lives |  | 7 years |  |
Core Deposit Intangibles [Member] | Maximum [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Intangible assets, Estimated useful lives |  | 10 years |  |
Customer Relationship Intangibles [Member] | Minimum [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Intangible assets, Estimated useful lives |  | 3 years |  |
Customer Relationship Intangibles [Member] | Maximum [Member] |  |  |  |
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items] |  |  |  |
Intangible assets, Estimated useful lives |  | 18 years |  |

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