Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 2,960.0	$ 4,008.7
Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	0.00%
Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for OREO Level 3 Fair Value	(65.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(37.00%)
Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	(22.00%)
Discount Rate for OREO Level 3 Fair Value	(27.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(3.00%)
Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	$ (13.7)	(2.8)
Other real estate owned	(1.2)
Long-lived assets to be disposed of by sale	(0.2)	(0.2)
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	2,960.0	4,008.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0.0	0.0
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	2,960.0	4,008.7
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0.0	0.0
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	$ (27.6)	(14.7)
Measurement Input, Discount Rate [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for OREO Level 3 Fair Value	(8.00%)
Discount Rate for Assets HFS Level 3 Fair Value	0.00%
Measurement Input, Discount Rate [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	(56.00%)
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 3.2	0.2
Other Debt Obligations [Member] | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	3.2	0.2
Other Debt Obligations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0.0	0.0
Other Debt Obligations [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	3.2	0.2
Other Debt Obligations [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0.0	0.0
Loans And Leases Receivable, Impaired Loans [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	0.0	0.0
Loans And Leases Receivable, Impaired Loans [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	0.0	0.0
Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	(27.6)
Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	(27.6)	$ (14.7)
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0.18
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0.09
Loans And Leases Receivable, Other Real Estate Owned [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	0.0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	0.0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	(2.2)
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	(2.2)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	0.0	$ 0.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	0.0	0.0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	(6.2)	(5.3)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	$ (6.2)	$ (5.3)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0