Investments and Cash - Narrative (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Government Agency Obligations as a Percentage of Total Mortgage Backed Securities	44.00%	50.00%
Percentage of decline in market value of security below amortized cost, considered for assessing impairment of investments	20.00%
Continuous period of decline in market value of security, considered for assessing impairment of investments	12 months
Accrued investment income	98	93
Number of outside managers managing investment portfolio	4
Number of securities with unrealized losses greater than 10% of book value for 12 months or more	3
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more	15
Assets held-in-trust	268us-gaap_AssetsHeldInTrust	396us-gaap_AssetsHeldInTrust
Fair market value of company's pledged securities	376	677
Investment in Portfolio Funding Company LLC I
Investment [Line Items]
Percentage of an equity investment acquired in a restructuring of an insured CDS	50.00%
Investments [Member] \| Available-for-sale Securities [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	99.00%
Investments [Member] \| Internally Managed Portfolio [Member]
Investment [Line Items]
Percentage of installment premiums denominated in currencies other than the U.S. dollar	8.00%	9.00%

Represents the number of securities which had unrealized losses greater than 10% of book value for 12 months or more.

No definition available.

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- Definition

This item represents the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for those securities which are categorized neither as held-to-maturity nor trading securities and which have unrealized losses greater than 10% of book value.

+ References

No definition available.

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- Definition

Represents the continuous period of decline in the market value of a security by any percentage below the amortized cost considered for the assessment of the impairment of investments.

+ References

No definition available.

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- Definition

Represents government agency obligations as a percentage of the aggregate amortized cost of mortgage backed securities.

+ References

No definition available.

+ Details

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- Definition

Represents the minimum percentage of decline in the market value of securities below the amortized cost considered for the assessment of the impairment of investments.

+ References

No definition available.

+ Details

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- Definition

Number of Outside Managers Managing Investment Portfolio

+ References

No definition available.

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- Definition

Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.3)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 3

-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 4

-Article 7

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- Definition

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(b))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959

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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6404-108592

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 20

-URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13531-108611

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-Name Accounting Standards Codification

-Topic 275

-SubTopic 10

-Section 50

-Paragraph 18

-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 323

-SubTopic 10

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

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