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Liabilities for Outstanding Claims Losses and Loss Adjustment Expenses - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended						6 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Reinsurance Pool Business	Dec. 31, 2011 Reinsurance Pool Business	Dec. 31, 2010 Reinsurance Pool Business	Dec. 31, 2011 Chaucer Business	Dec. 31, 2012 Chaucer Business	Dec. 31, 2011 Chaucer Business
Liabilities For Outstanding Claims, Losses And Loss Adjustment Expenses [Line Items]
Decrease in reserves after re-estimation	$ 15.8	$ 103.3	$ 111.1
Favorable loss and LAE reserve development							35.5	72.6	35.5
Favorable development included in unallocated loss adjustment expense reserves			9.8
Loss and LAE reserves of asbestos and environmental damage liability	60.5	59.8	63.9
Net loss and LAE reserves for asbestos and environmental damage liability	9.8	10.0	10.1
Reinsurance of loss and LAE reserves	20.4	18.7	19.9
Gross loss and LAE reserves for asbestos and environmental damage liability				$ 30.3	$ 31.1	$ 33.9

Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, before estimated recoveries from reinsurers.

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- Definition

Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries.

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- Definition

Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, net of estimated reinsurance recoveries.

+ References+ Details