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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