Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014
Significant Accounting Policies [Line Items]
Premiums Earned and Associated Tax Accruals Adjustment Amount	$ 20
Options, life of award	10 years
Alternative funds
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	1 month
Private investment funds
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	3 months
Operating affiliates
Significant Accounting Policies [Line Items]
Alternative investments, timelag for reporting	3 months
Investments in the common stock of a corporation
Significant Accounting Policies [Line Items]
Threshold for signicant influence, percentage	20.00%
Investments in closed end funds, limited partnerships, LLCs or similar investment vehicles
Significant Accounting Policies [Line Items]
Threshold for signicant influence, percentage	3.00%
Maximum
Significant Accounting Policies [Line Items]
Hedge effectiveness, percentage	125.00%
Vesting period required	4 years
Minimum
Significant Accounting Policies [Line Items]
Hedge effectiveness, percentage	80.00%
Vesting period required	3 years

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The period by which the reporting for a given investment lags behind the balance sheet date.

+ References

No definition available.

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- Definition

Hedge effectiveness, percentage.

+ References

No definition available.

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- Definition

The amount of adjustment, net of tax, to premiums earned and associated tax accruals related to reinstatement premiums due under assumed reinsurance contracts arising from unpaid losses and loss expenses reported in a prior period.

+ References

No definition available.

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- Definition

The percentage ownership threshold in a given investment, if exceeded, indicates that significant influence generally exists.

+ References

No definition available.

+ Details

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- Details

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