Merger with Transatlantic - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			1 Months Ended	3 Months Ended		7 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Mar. 06, 2012	Mar. 06, 2012 Transatlantic	Sep. 30, 2012 Transatlantic Location	Dec. 31, 2011 Transatlantic	Sep. 30, 2012 Transatlantic Location	Sep. 30, 2012 Transatlantic Location	Sep. 30, 2012 Transatlantic Pro Forma Catastrophe	Sep. 30, 2011 Transatlantic Pro Forma Catastrophe	Sep. 30, 2012 Transatlantic Pro Forma Catastrophe Prior to acquisition date	Sep. 30, 2012 Transatlantic Pro Forma Costa Concordia	Sep. 30, 2012 Transatlantic Pro Forma Costa Concordia Prior to acquisition date	Mar. 06, 2012 Before Acquisition Person	Mar. 06, 2012 Additional Person
Business Acquisition [Line Items]
Value of merger consideration paid per share to acquiree stockholders						$ 61.14
Total value of consideration paid to acquiree stockholders						$ 3,500,000,000
Per share exchange ratio of 0.145 shares of Common Stock per share of Old Transatlantic common stock						0.145
Per share value of consideration paid to former Transatlantic stockholders not dependent on Alleghany's common stock price						$ 14.22
Portion of purchase price based on cash consideration						816,000,000
Shares of Common Stock issued						8,360,959
Number of members of board of directors																11	3
Merger-related costs								19,300,000		33,800,000
Merger-related costs incurred, Investment bank fee										18,000,000
Percentage of stockholders' pre-merger voting interest after merger with Transatlantic					51.00%
Voting interest percentage						49.00%
Gain on bargain purchase			494,940,000			494,900,000
Aggregate value of outstanding stock options settled						11,100,000
Value of acquiree's former restricted stock units as of the acquisition date						49,500,000
Restricted stock units paid by Alleghany										1,100,000
Period to pay acquiree's former restricted stock units as of the acquisition date										3 years
Number of states entity operates							50		50	50
Pre-tax losses	$ 672,984,000	$ 121,777,000	$ 1,587,815,000	$ 315,418,000			$ 562,500,000		$ 1,271,000,000		$ 54,000,000	$ 682,500,000	$ 53,700,000	$ 19,000,000	$ 15,800,000