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Loss and Loss Adjustment Expense Reserves Details (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 One Beacon	Sep. 30, 2012 One Beacon	Sep. 30, 2013 One Beacon	Sep. 30, 2012 One Beacon	Sep. 30, 2013 Sirius Group [Member]	Sep. 30, 2012 Sirius Group [Member]	Sep. 30, 2013 Sirius Group [Member]	Sep. 30, 2012 Sirius Group [Member]	Mar. 31, 2012 Scandinavian Reinsurance Company [Member]
Loss and Loss Adjustment, Expense Reserves [Line Items]
Net favorable and unfavorable loss reserve development	$ (11.6)	$ (10.1)	$ (25.6)	$ (21.2)	$ 3.8	$ (2.3)	$ (0.1)	$ (7.6)	$ (15.4)	$ (7.8)	$ (25.5)	$ (13.6)
Liability for Asbestos and Environmental Claims, Net, Period Increase (Decrease)										33.0		45.0
Accretion of fair value adjustment to net loss and LAE reserves	0.2	1.1	1.5	9.4
Accretion of fair value adjustment to net loss and LAE reserves increase													5.0
Adjustment to loss and loss adjustment expense reserves, before tax	$ 4.2		$ 4.2

X

- Definition

The increase (decrease) during the reporting period in asbestos and environmental claims reserve, net of reinsurance recoveries.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (b),(c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

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X

- Definition

Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs.

+ References

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

-Publisher SEC

-Name Financial Reporting Release (FRR)

-Number 403

-Paragraph 04

-Subparagraph b



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

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X

- Definition

Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting.

+ References

No definition available.

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X

- Definition

Represents the amount of accreted fair value adjustment to loss and LAE reserves related to purchase accounting.

+ References

No definition available.

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- Definition

Represents the amount of pre-tax unaccreted fair value adjustment to loss and LAE reserves related to purchase accounting.

+ References

No definition available.

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