Fair Value (Quantitative Information about Level 3 Fair Value Measurements-Recurring) (Details) - Level 3 - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value - assets	$ 658.8	$ 769.0
Estimated fair value liabilities	(124.4)	(120.6)
FDIC True-up liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value liabilities	$ (64.6)	$ (61.9)
Fair Value Inputs, Discount Rate (percentage)	2.60%	3.20%
Consideration holdback liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value liabilities	$ (46.0)	$ (47.2)
Derivative liabilities — non-qualifying
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value liabilities	(13.8)	(11.5)
Securities — AFS
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value - assets	411.0	485.5
Securities carried at fair value with changes recorded in net income
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value - assets	247.7	$ 283.5
Derivative assets — non qualifying
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Estimated fair value - assets	$ 0.1
Minimum \| Consideration holdback liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)		1.30%
Fair value measurements, Payment Probability (percentage)	0.00%	0.00%
Minimum \| Securities — AFS
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	0.00%	0.00%
Fair Value Inputs, Prepayment Rate (percentage)	2.30%	3.20%
Fair Value Inputs, Default Rate (percentage)	0.00%	0.00%
Fair Value Inputs, Loss Severity (percentage)	0.60%	1.00%
Minimum \| Securities carried at fair value with changes recorded in net income
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	0.80%	0.00%
Fair Value Inputs, Prepayment Rate (percentage)	9.30%	6.10%
Fair Value Inputs, Default Rate (percentage)	2.70%	1.90%
Fair Value Inputs, Loss Severity (percentage)	14.30%	22.20%
Minimum \| Derivative assets — non qualifying
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Borrower Rate (percentage)	2.80%
Maximum \| Consideration holdback liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)		4.00%
Fair value measurements, Payment Probability (percentage)	100.00%	100.00%
Maximum \| Securities — AFS
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	41.10%	96.40%
Fair Value Inputs, Prepayment Rate (percentage)	22.60%	21.20%
Fair Value Inputs, Default Rate (percentage)	6.60%	9.00%
Fair Value Inputs, Loss Severity (percentage)	70.60%	79.80%
Maximum \| Securities carried at fair value with changes recorded in net income
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	45.10%	34.60%
Fair Value Inputs, Prepayment Rate (percentage)	19.40%	16.20%
Fair Value Inputs, Default Rate (percentage)	9.60%	8.10%
Fair Value Inputs, Loss Severity (percentage)	43.20%	44.70%
Maximum \| Derivative assets — non qualifying
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Borrower Rate (percentage)	4.70%
Weighted Average \| FDIC True-up liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	2.60%	3.20%
Weighted Average \| Consideration holdback liability
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)		2.10%
Fair value measurements, Payment Probability (percentage)	40.00%	40.90%
Weighted Average \| Securities — AFS
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	4.40%	5.50%
Fair Value Inputs, Prepayment Rate (percentage)	8.90%	8.80%
Fair Value Inputs, Default Rate (percentage)	3.60%	3.90%
Fair Value Inputs, Loss Severity (percentage)	37.60%	36.30%
Weighted Average \| Securities carried at fair value with changes recorded in net income
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Discount Rate (percentage)	4.20%	5.60%
Fair Value Inputs, Prepayment Rate (percentage)	12.30%	11.90%
Fair Value Inputs, Default Rate (percentage)	4.70%	4.60%
Fair Value Inputs, Loss Severity (percentage)	26.70%	25.80%
Weighted Average \| Derivative assets — non qualifying
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fair Value Inputs, Borrower Rate (percentage)	3.70%

Fair value measurements, borrower rate

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Fair value measurements, payment probability

+ References

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- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value on a recurring basis by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ References

No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Loss, including interest and principal, incurred on a defaulted security, expressed as a percentage of the original principal balance, used as an input to measure fair value.

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- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

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- Definition

Percentage of likelihood a loan will not be repaid and instead default, used as an input to measure fair value.

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- Definition

Aggregation of the liabilities reported on the balance sheet measured at fair value on a recurring basis by the entity.

+ References

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