Third Party Reinsurance (One Beacon) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 31.7	$ 23.6
Reinsurance recoverable on unpaid losses	427.5	483.9
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	20.9	12.2
Reinsurance recoverable on unpaid losses	$ 147.1	$ 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%
OneBeacon
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Excess Retention, Amount Reinsured	$ 130.0
Reinsurance Retention Policy, Amount Retained	$ 20.0
OneBeacon \| Combination One
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	95.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation	$ 10.0
OneBeacon \| Minimum \| Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	3.0
OneBeacon \| Minimum \| Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	3.0
OneBeacon \| Maximum \| Property [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	5.0
OneBeacon \| Maximum \| Health Care [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance Retention Policy, Amount Retained	$ 5.0
OneBeacon \| Combination Two
Reinsurance Retention Policy [Line Items]
Percentage of par value of policy reinsured	100.00%
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation	$ 100.0

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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