Level 3 Assets - Unobservable inputs (Details) (Level 3, USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
U.S. government and agency
Fair Value, Option, Quantitative Disclosures [Line Items]
U.S. government and agency	80.2	58.8
U.S. government and agency \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.02%	1.07%
U.S. government and agency \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.64%	5.00%
U.S. government and agency \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.75%	3.18%
State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
State and political subdivision	65.9	45.4
State and political subdivision \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.34%	2.44%
State and political subdivision \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	4.50%	5.79%
State and political subdivision \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.33%	3.88%
Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Corporate	1,053.3	874.8
Corporate \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	0.93%	1.06%
Corporate \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	6.88%	6.75%
Corporate \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.35%	3.82%
Other asset-backed (“ABS”)
Fair Value, Option, Quantitative Disclosures [Line Items]
Other asset-backed	9.8	11.3
Default rate		2.53%
Default rate term 2		0.37%
Recovery rate		10.00%
Prepayment rate		2.00%
Other asset-backed (“ABS”) \| Minimum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.83%	2.10%
Other asset-backed (“ABS”) \| Maximum
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.01%	3.41%
Other asset-backed (“ABS”) \| Weighted Average
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.01%	2.30%
Fair value investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value investments	1.0	0.8
Default rate	0.17%	0.25%
Recovery rate	44.00%	45.00%

Fair Value Inputs, Probability of Default, Term Two

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- Definition

Fair Value Inputs, Recovery Rate

+ References

No definition available.

+ Details

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- Definition

Fair Value Inputs, Yield Rate

+ References

No definition available.

+ Details

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- Definition

No authoritative reference available.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

No authoritative reference available.

+ References

No definition available.

+ Details

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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.

+ 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=36462937&loc=d3e19207-110258

+ Details

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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.

+ References

+ Details

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- Details

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- Definition

The fair value as of the balance sheet date of firm holdings in fixed income or other debt securities issued by a corporation or other issuer not elsewhere specified in the taxonomy. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

+ Details

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- Definition

The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

+ Details

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- Definition

Bonds or similar securities issued by state, city, or local governments or the agencies operated by state, city, or local governments which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

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