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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details) |  |  | 12 Months Ended
--- | --- | --- | ---
Apr. 30, 2020 USD ($) | Feb. 01, 2019 USD ($) shares | Dec. 31, 2020 USD ($) item loan shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Accounting Policies [Line Items] |  |  |  |  |
Number of bank branches | item |  |  | 134 |  |
Number of ATM's | item |  |  | 155 |  |
Accruals for uncertain tax positions |  |  | $ 0 | $ 0 |
Impairment charges for goodwill | $ 0 |  | 0 |  |
Available for sale securities, allowance for credit losses |  |  | 0 |  |
Goodwill |  |  | 935,560,000 | 935,560,000 |
Loans held for investment, accrued interest receivable |  |  | 56,700,000 |  |
HTM securities, Accrued interest receivable |  |  | 6,800,000 |  |
AFS securities, accrued interest receivable |  |  | 11,900,000 |  |
Cash paid in acquisition |  |  | 0 | 12,000 | $ 14,304,000
Liabilities for post retirement benefits payable to other beneficiaries |  |  | $ 13,800,000 | $ 12,100,000 |
Stock options granted | shares |  |  | 0 |  |
HTM securities, additional reserve for allowance for credit losses |  |  | $ 0 |  |
Minimum |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |
Intangible assets, amortization period (years) |  |  | 4 years |  |
Estimated useful life of bank premises |  |  | 3 years |  |
Percentage of fair value of the derivative instruments for the hedge to be highly effective |  |  | 80.00% |  |
Maximum |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |
Intangible assets, amortization period (years) |  |  | 10 years |  |
Estimated useful life of bank premises |  |  | 50 years |  |
Percentage of fair value of the derivative instruments for the hedge to be highly effective |  |  | 125.00% |  |
Access National Bank |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |
Number of shares equivalent to each share of acquired entity | shares |  | 0.75 |  |  |
Company's common shares issued | shares |  | 15,842,026 |  |  |
Cash paid in acquisition |  | $ 12,000 |  |  |
Paycheck Protection Program (PPP) |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |
Loans issued under the CARES Act |  |  | $ 1,700,000,000 |  |
Loans, recorded investment under the CARES Act |  |  | 1,200,000,000 |  |
Loan origination fees Under the CARES Act |  |  | $ 17,600,000 |  |
Number of loans forgiven under the CARES Act | loan |  |  | 3,100 |  |
Loans forgiven under the CARES Act |  |  | $ 429,300,000 |  |
Paycheck Protection Program (PPP) | Minimum |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |
Number of loans processed under the CARES Act | loan |  |  | 11,000 |  |

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