Stockholders Equity - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended
	Feb. 29, 2008	Jun. 30, 2006	Jun. 30, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Class of Stock [Line Items]
Mandatory convertible preferred stocks offered		1,132,000
Mandatory convertible preferred stock, offering Price		$ 264.60
Proceed from issuance of mandatory convertible preferred stock		$ 290,400,000
Weighted-average price assigned per share of common stock upon conversion of preferred stock			$ 260.9733
Common stocks issued upon conversion of preferred stock			698,009
Preferred Stocks converted			688,621
Common shares issued for each converted preferred stock			1.0139
Convertable preferred stock, stated dividend rate		5.75%
Aggregate amount of common stock authorized for repurchase	300,000,000				300,000,000
Treasury stock purchase, shares				399,568	285,056	295,463
Treasury stock purchase				120,325,000	83,135,000	75,856,000
Treasury stock purchase, average price per share				$ 301.14	$ 291.64	$ 256.73
Preferred stock repurchased						442,998
Convertible preferred stock acquisition						117,358,000
Preferred stock repurchase, average price per share						$ 264.92
Total stockholders' equity				2,925,677,000	2,908,868,000	2,717,521,000	2,646,689,000
Aggregate total equity of Alleghany's insurance operating units				1,850,000,000
Amount Available for dividends without prior approval of the applicable insurance regulatory authorities				35,900,000
Statutory net income of Alleghany's insurance operating units				147,400,000	219,200,000
Combined statutory capital and surplus of Alleghany's insurance operating unit				1,600,000,000	1,600,000,000
Subsidiaries
Class of Stock [Line Items]
Amount available for dividends or advances to Alleghany at the parent level				835,300,000
Amount unavailable for dividends or advances to Alleghany from the subsidiaries				$ 1,820,000,000