Third Party Reinsurance (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	$ 16.6	$ 7.5
Reinsurance recoverable on unpaid losses	150.9	186.5
OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Reinsurance recoverable on paid losses	16.6	7.5
Reinsurance recoverable on unpaid losses	$ 150.4	$ 186.0
OneBeacon [Member]
Reinsurance Retention Policy [Line Items]
Percentage of Quota Share Reinsurance Agreement	50.00%

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.

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Represents the percentage ceded under quota share reinsurance agreement.

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