Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 310

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 7

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- Definition

Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 6

-Article 7

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 20

-Section 50

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.(a),6)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Amount after valuation allowance of reinsurance recoverables on paid losses.

+ References

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- Definition

Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

+ References

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- Definition

Percentage of total reinsurance recoverables.

+ References

No definition available.

+ Details

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- Definition

Stated amount that is the upper retention limit per entity policy.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	duration

- Definition

Amount of risk reinsured in excess of stated retention per company policy.

+ References

No definition available.

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- Definition

Percentage of risk reinsured on insurance products per entity policy.

+ References

No definition available.

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- Definition

The number of ratings in the A.M. Best rating system.

+ References

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- Definition

The respective third party reinsurers' percentage share of the total paid and unpaid reinsurance recoverables.

+ References

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Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified								9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2013 OneBeacon Rating	Sep. 30, 2013 OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Minimum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Maximum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer One [Member] OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer One [Member] Minimum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer One [Member] Maximum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Two [Member] OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Two [Member] Minimum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Two [Member] Maximum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Three [Member] OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Three [Member] Minimum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Layer Three [Member] Maximum OneBeacon Property Catastrophe Reinsurance [Member]	Sep. 30, 2013 Hannover Ruversich [Member] AM Best, A+ Rating [Member] OneBeacon	Sep. 30, 2013 Munich Reinsurance America [Member] AM Best, A+ Rating [Member] OneBeacon	Sep. 30, 2013 Platinum Underwriters Re [Member] AM Best, A Rating [Member] OneBeacon	Sep. 30, 2013 Swiss Re Group AM Best, A+ Rating [Member] OneBeacon	Sep. 30, 2013 Hartford Steam Boiler [Member] AM Best, A++ Rating [Member] OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 18.8		$ 17.9				$ 5.9
Reinsurance recoverables																				10.3	6.6	5.6	4.6	4.8
Reinsurance recoverable on unpaid losses	416.0	388.6	429.1	356.8	2,369.4	2,507.3
Reinsurance recoverable on unpaid losses	416.0		429.1				80.2
Percentage of total paid and unpaid reinsurance recoverables							90.00%
Number of ratings in A.M. Best ratings							16
Property Catastrophe Reinsurance Program Retained Amount									20.0	150.0
Property Catastrophe Reinsurance Program, Second Layer								$ 130.0				$ 20.0	$ 30.0		$ 30.0	$ 70.0		$ 70.0	$ 150.0
Property Catastrophe Reinsurance Program Retention Percentage											50.00%			10.00%			5.00%
Percentage of total paid and unpaid reinsurance recoverables																				12.00%	8.00%	7.00%	5.00%	6.00%