Investments - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2012 Investment	Mar. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sales of AFS securities	$ 700,000,000	$ 400,000,000
Pre tax gain on sale of holdings in Exxon Mobil	63,100,000
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.
OTTI unrealized losses that were charged against earnings	1,778,000
Number of debt and equity securities in an unrealized loss position	1,038
Percentage of debt securities issued with credit rating below investment grade or not rated	0.90%
Statutory deposit, investments at fair value	953,500,000
Minimum
Schedule of Available-for-sale Securities [Line Items]
Percentage of impaired equity securities in unrealized loss position above amortized cost	50.00%
Percentage of equity securities in unrealized loss position above amortized cost under impairment test	20.00%
Equity securities | Energy Sector Business
Schedule of Available-for-sale Securities [Line Items]
OTTI unrealized losses that were charged against earnings	900,000
Debt securities
Schedule of Available-for-sale Securities [Line Items]
OTTI unrealized losses that were charged against earnings	$ 900,000
Number of debt and equity securities in an unrealized loss position	12