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, before estimated recoveries from reinsurers.

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- Definition

A description of the accrued obligation to policyholders pertaining to insured events pertaining to long-duration contracts, which can be viewed as either (a) the present value of future benefits to be paid to or on behalf of policyholders and expenses less the present value of future net premiums payable under the insurance contracts or (b) the accumulated amount of net premiums already collected less the accumulated amount of benefits and expenses already paid to or on behalf of policyholders.

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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jul. 31, 2005
Loss Contingencies [Line Items]
Reserves for unpaid loss and loss adjustment expenses, gross	$ 13.9	$ 14.1
Reserves for unpaid loss and loss adjustment expenses, net	13.8	14.0
Indemnification obligation	150
Indemnification obligation terms	Under the terms of the Stock Purchase Agreement, with respect to products liability claims arising in respect of events occurring during the period prior to Alleghany’s acquisition of World Minerals, Alleghany will provide indemnification at a rate of 100 percent for the first $100.0 million of losses arising from and at a rate of 50 percent for the next $100.0 million of such losses, so that Alleghany’s maximum indemnification obligation in respect of products liability claims relating to such period of time is $150.0 million.
Reserve for Products Liability Indemnification	0.2	0.2	0.6
Concentration of market risk related to available-for-sale equity securities portfolio	1,024.9	1,004.8
Exxon Mobil Corporation
Loss Contingencies [Line Items]
Concentration of market risk related to available-for-sale equity securities portfolio	$ 651.0
Claims Related to Events Before Acquisition \| First $100 Million Of Losses
Loss Contingencies [Line Items]
Indemnification percentage	100.00%
Claims Related to Events Before Acquisition \| For the Next $100 Million of Losses
Loss Contingencies [Line Items]
Indemnification percentage	50.00%