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Summary of Significant Accounting Policies (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended		1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2001 OneBeacon Ltd. Purchase price adjustments	Jun. 02, 2001 OneBeacon Ltd. Purchase price adjustments	Dec. 31, 2004 Sirius International Purchase price adjustments
Insurance and reinsurance operations
Average discount rate used to discount long-term workers compensation loss and LAE reserves (as a percent)	4.50%	5.00%
Amount of discount on workers compensation loss and LAE reserves	$ 108.3	$ 119.4
Purchase accounting adjustments
Adjustment to fair value loss and LAE reserves				646.9	58.1
Adjustment to fair value of reinsurance recoverables related to loss and LAE reserves				346.9
Net purchase accounting adjustment to net loss reserves			$ 300.0

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- Definition

Represents the net amount of the purchase accounting adjustments to loss reserves and reinsurance recoverables.

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Purchase accounting adjustment to fair value for loss and LAE reserves.

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Purchase accounting adjustment to fair value for reinsurance recoverables.

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- Definition

Represents the amount by which workers compensation loss and LAE reserves are discounted.

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- Definition

Represents the average discount rate used to discount certain of the long-term workers compensation loss and LAE reserves, which is determined based on the various assumptions including consideration of when the claims will be settled.

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No definition available.

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