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Significant Accounting Policies - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 USD ($) | Dec. 31, 2020 USD ($) segment shares | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Trust assets |  |  |  |
Assets under care |  | $ 10,200.0 | $ 9,700.0 |
Allowance-debt securities available for sale |  |  |  |
Accrued interest receivable for debt securities available for sale debt |  | 6.8 |  |
Loan servicing and portfolio loans |  |  |  |
Unpaid principal balances of loans serviced |  | 2,200.0 |  |
Impairment charges |  | 0.6 | 0.0 |
Goodwill |  |  |  |
Goodwill impairment |  | 0.0 | 0.0 |
Other asset investments |  |  |  |
Carrying value of VIE |  | 37.1 |  |
Cash surrender value of bank owned life insurance |  |  |  |
Accrued liability |  | 5.6 | 5.7 |
Income taxes |  |  |  |
Valuation allowance |  | 0.0 | 0.0 |
Accruals for payments of interest and penalties |  | 0.0 | 0.0 |
Stock-based employee compensation |  |  |  |
Compensation expense recognized |  | $ 7.1 | $ 3.8 | $ 3.5
Segment disclosure |  |  |  |
Number of operating segments | segment |  | 3 |  |
Buildings and improvements | Minimum |  |  |  |
Premises and Equipment, net |  |  |  |
Estimated Useful Life |  | 3 years |  |
Buildings and improvements | Maximum |  |  |  |
Premises and Equipment, net |  |  |  |
Estimated Useful Life |  | 40 years |  |
Furniture and equipment | Minimum |  |  |  |
Premises and Equipment, net |  |  |  |
Estimated Useful Life |  | 3 years |  |
Furniture and equipment | Maximum |  |  |  |
Premises and Equipment, net |  |  |  |
Estimated Useful Life |  | 10 years |  |
Restricted stock unit |  |  |  |
Stock-based employee compensation |  |  |  |
Number of shares of common stock per award | shares |  | 1 |  |
Restricted stock unit | Minimum |  |  |  |
Stock-based employee compensation |  |  |  |
Requisite service periods |  | 1 year |  |
Restricted stock unit | Maximum |  |  |  |
Stock-based employee compensation |  |  |  |
Requisite service periods |  | 5 years |  |
Deferred stock units |  |  |  |
Stock-based employee compensation |  |  |  |
Number of shares of common stock per award | shares |  | 1 |  |
Requisite service periods |  | 30 days |  |
Vesting period |  | 1 year |  |
Performance based restricted stock unit |  |  |  |
Stock-based employee compensation |  |  |  |
Number of shares of common stock per award | shares |  | 1 |  |
Compensation expense recognized | $ 0.3 |  |  |

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