Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Investment	Sep. 30, 2011	Sep. 30, 2012 Investment	Sep. 30, 2011
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of AFS securities	$ 600,000,000	$ 300,000,000	$ 1,900,000,000	$ 800,000,000
Pre-tax net realized capital gains	12,388,000	22,706,000	119,829,000	63,888,000
Securities impairment test description			Management’s assessment of equity securities initially involves an evaluation of all securities that are in an unrealized loss position, regardless of the duration or severity of the loss, as of the applicable balance sheet date. Such initial review consists primarily of assessing whether: (i) there has been a negative credit or news event with respect to the issuer that could indicate the existence of an OTTI; and (ii) Alleghany has the ability and intent to hold an equity security for a period of time sufficient to allow for an anticipated recovery (generally considered to be less than one year from the balance sheet date). To the extent that an equity security in an unrealized loss position is not impaired based on the initial review described above, Alleghany then further evaluates such equity security and deems it to be other-than-temporarily impaired if it has been in an unrealized loss position for twelve months or more or if its unrealized loss position is greater than 50 percent of its cost, absent compelling evidence to the contrary. Alleghany then evaluates those equity securities where the unrealized loss is 20 percent or more of cost as of the balance sheet date or which have been in an unrealized loss position continuously for six months or more preceding the balance sheet date. This evaluation takes into account quantitative and qualitative factors in determining whether such securities are other-than-temporarily impaired including: (i) market valuation metrics associated with the equity security (e.g., dividend yield and price-to-earnings ratio); (ii) current views on the equity security, as expressed by either Alleghany’s internal stock analysts and/or by third party stock analysts or rating agencies; (iii) and discrete credit or news events associated with a specific company, such as negative news releases and rating agency downgrades with respect to the issuer of the investment. Debt securities in an unrealized loss position are evaluated for OTTI if they meet any of the following criteria: (i) they are trading at a 20 percent discount to amortized cost for an extended period of time (nine consecutive months or longer); (ii) there has been a negative credit or news event with respect to the issuer that could indicate the existence of an OTTI; (iii) Alleghany intends to sell or it is more likely than not that Alleghany will sell the debt security before recovery of its amortized cost basis; and (iv) Alleghany may not realize a full recovery on its investment, regardless of the occurrence of one or more of the foregoing events. If Alleghany intends to sell, or it is more likely than not that Alleghany will sell, a debt security before recovery of its amortized cost basis, the total amount of the unrealized loss position is recognized as an OTTI loss in earnings. To the extent that a debt security that is in an unrealized loss position is not impaired based on the preceding, Alleghany will consider a debt security to be impaired when it believes it to be probable that Alleghany will not be able to collect the entire amortized cost basis. For debt securities in an unrealized loss position as of the end of each quarter, Alleghany develops a best estimate of the present value of expected cash flows on a security by security basis. If the results of the cash flow analysis indicate Alleghany will not recover the full amount of its amortized cost basis in the investment, Alleghany records an OTTI loss in earnings equal to the difference between the present value of expected cash flows and the amortized cost basis of the security. If applicable, the difference between the total unrealized loss position on the security and the OTTI loss recognized in earnings is the non-credit related portion and is recorded as a component of other comprehensive income. In developing the cash flow analyses for debt securities, Alleghany considers various factors for the different categories of debt securities. For municipal bonds, Alleghany takes into account the taxing power of the issuer, source of revenue, credit risk and credit enhancements and pre-refunding. For mortgage and asset-backed securities, Alleghany discounts its best estimate of future cash flows at an effective rate equal to the original effective yield of the security or, in the case of floating rate securities, at the current coupon. Alleghany’s models include assumptions about prepayment speeds, default and delinquency rates, and underlying collateral (if any), as well as credit ratings, credit enhancements and other observable market data. For corporate bonds, Alleghany reviews business prospects, credit ratings and available information from asset managers and rating agencies for individual securities.
Other than temporary impairment losses charged against earnings		2,756,000	2,907,000	2,756,000
Number of debt and equity securities in an unrealized loss position	204		204
Percentage of debt securities issued with credit rating below investment grade or not rated	1.40%		1.40%
Statutory deposit, investments at fair value	973,600,000		973,600,000
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment losses charged against earnings			1,700,000	2,600,000
Equity Securities | Minimum
Schedule of Available-for-sale Securities [Line Items]
Percentage of unrealized loss to cost where a security would be deemed to be other than temporarily impaired, absent compelling evidence to the contrary			50.00%
Percentage of unrealized loss to cost where a security would be evaluated for other than temporarily impairment			20.00%
Equity Securities | Exxon Mobil Corporation
Schedule of Available-for-sale Securities [Line Items]
Pre-tax net realized capital gains			63,100,000
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Other than temporary impairment losses charged against earnings			$ 1,200,000	$ 200,000
Number of securities in an unrealized loss position for 12 months or more	9		9