Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	29.8	$ 23.6
Reinsurance recoverable on unpaid losses	436.9	483.9
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	18.2	12.2
Reinsurance recoverable on unpaid losses	141.6	$ 161.6
Crop insurance [Member] \| OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	52.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	98.00%
Percentage of Quota Share Reinsurance Agreement	30.00%
Crop-hail insurance [Member] \| OneBeacon
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	80.00%
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage	100.00%

Amount after valuation allowance of reinsurance recoverables on paid losses.

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

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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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Percentage of risk reinsured on insurance products per entity policy.

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