Unaudited Pro Forma Consolidated Information (Detail) (TransRe, USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
TransRe
Business Acquisition, Pro Forma Information [Line Items]
Total revenues	$ 5,043.4	[1]	$ 5,327.0	[1]
Net earnings (losses)	$ 270.0	[1],[2]	$ (15.2)	[1],[2]
Basic earnings per share attributable to Alleghany stockholders	$ 17.48		$ (0.89)

[1] Among other adjustments, reflects an increase in amortization expense on TransRe's AFS debt securities resulting from the adjustment of amortized cost to their fair value as of the Pro Forma TransRe Acquisition Dates. Such adjustment reduced the net investment income of TransRe.

[2] Among other adjustments, reflects adjustments to amortization expense on the TransRe Senior Notes resulting from the adjustment to their fair value as of the Pro Forma TransRe Acquisition Dates. Such adjustment reduced the interest expense of TransRe. See Note 8(b) for additional information on the TransRe Senior Notes. Also reflects adjustments to amortization expense associated with intangible assets arising from the merger.