Senior Notes and Credit Agreement - Additional Information (Detail) (USD $)	1 Months Ended		12 Months Ended
	Sep. 20, 2010	Sep. 09, 2010	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Senior notes, face value	$ 300,000,000
Line of credit facility borrowing capacity description		Alternate Base Rate Borrowings under the Credit Agreement bear interest at (x) the greatest of (a) the administrative agent’s prime rate, (b) the federal funds rate plus 0.5 percent or (c) an adjusted London Interbank Overnight (“LIBO”) rate for a one month interest period on such day plus 1 percent, plus (y) a specified margin (currently 0 basis points for Tranche A and 125 basis points for Tranche B). Eurodollar Borrowings under the Credit Agreement bear interest at an adjusted LIBO Rate for the interest period in effect plus a specified margin (currently 75 basis points for Tranche A and 225 basis points for Tranche B). The Credit Agreement requires that all loans be repaid in full no later than the Maturity Date. The Credit Agreement also requires Alleghany to pay a commitment fee each quarter in a range of between one fifth and one-half of one percent per annum, in each case based upon Alleghany’s credit ratings, on the daily unused amount of the Commitments of the relevant Tranche.
Line of credit facility, term		3 years
Debt, maturity date	Sep. 15, 2020	Sep. 09, 2013
Senior notes, dates of interest payable	Interest is payable semi-annually on March 15 and September 15 of each year.
Borrowing under line of credit, applicable rate above federal funds rate		0.50%
Senior notes, issuance rate	99.63%
Borrowing under line of credit, applicable rate above LIBOR rate		1.00%
Proceeds from issuance of Senior Notes				298,893,000
Senior notes, effective yield	5.67%
Underwriting discount, commissions and other expense	2,800,000
Senior notes, interest rate	5.625%
Revolving credit facility, maximum borrowing capacity		100,000,000
Line of credit facility covenant terms			The Credit Agreement contains representations, warranties and covenants customary for bank loan facilities of this nature. In this regard, the Credit Agreement requires Alleghany to, among other things, (i) maintain a consolidated net worth of not less than the sum of (x) approximately $2.0 billion plus (y) 50 percent of Alleghany’s accumulated, consolidated net earnings earned in each fiscal quarter (if positive) commencing September 30, 2010 and (ii) maintain a ratio of total indebtedness to total capital as of the end of each fiscal quarter of not greater than 0.25 to 1.0.
Minimum consolidated net worth required as part of debt covenant			2,000,000,000
Maximum
Debt Instrument [Line Items]
Total indebtedness to total capital ratio, maximum per debt covenant			0.25
Senior Secured Credit Facility Tranche A Variable Rates
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity		50,000,000
Borrowing under line of credit, applicable basis points		0.00%
Senior Secured Credit Facility Tranche A Variable Rates | Euro Dollar Rate
Debt Instrument [Line Items]
Borrowing under line of credit, applicable basis points		0.75%
Senior Secured Credit Facility Tranche B Variable Rates
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity		$ 50,000,000
Borrowing under line of credit, applicable basis points		1.25%
Senior Secured Credit Facility Tranche B Variable Rates | Euro Dollar Rate
Debt Instrument [Line Items]
Borrowing under line of credit, applicable basis points		2.25%