Fair Value (Changes in Estimated Fair Value for Financial Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value	$ 247.7		$ 283.5
Derivative liabilities — non-qualifying
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(11.5)	$ (55.5)
Included in earnings	2.3	(7.2)
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	0.0	0.0
Ending Balance	(13.8)	(48.3)
FDIC True-up liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(61.9)	(56.9)
Included in earnings	2.7	4.4
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	0.0	0.0
Ending Balance	(64.6)	(61.3)
Consideration holdback liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(47.2)	(60.8)
Included in earnings	0.0	0.5
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	1.2	14.3
Ending Balance	(46.0)	(47.0)
Securities — AFS
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	485.5	567.1
Included in earnings	(1.4)	(4.6)
Included in comprehensive income	15.8	22.1
Impairment	(0.3)	(2.2)
Settlements	(88.6)	(72.0)
Ending Balance	411.0	510.4
Securities carried at fair value with changes recorded in net income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	283.5	339.7
Included in earnings	15.0	11.6
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	(50.8)	(50.0)
Ending Balance	247.7	301.3
FDIC Receivable
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0.6	54.8
Included in earnings	0.9	4.8
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	(1.1)	(10.3)
Ending Balance	0.4	49.3
Derivative assets — non qualifying
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0.0	0.0
Included in earnings	0.1	0.4
Included in comprehensive income	0.0	0.0
Impairment	0.0	0.0
Settlements	0.0	0.0
Ending Balance	$ 0.1	$ 0.4

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Impairment

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of investments in debt securities classified as trading.

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