Calculation of Total Purchase Price for Old Transatlantic as of Acquisition Date (Detail) (TransRe, USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended
	Mar. 06, 2012
TransRe
Business Acquisition [Line Items]
Shares of Old TransRe common stock outstanding as of the TransRe Acquisition Date	57,600,000
Multiplied by per share exchange ratio of 0.145 shares of Common Stock per share of Old TransRe common stock	0.145
Shares of Common Stock issued	8,360,959
Multiplied by the Acquisition Date closing price per share of Common Stock	$ 322.50	[1]
Portion of purchase price based on shares of Common Stock issued	$ 2,696.4
Adjustment to Old TransRe common stock	(200,000)	[2]
Old TransRe common stock, as adjusted	57,400,000
Multiplied by cash price per share component	$ 14.22
Portion of purchase price based on cash consideration	816.0
Total purchase price	$ 3,512.4

[1] As noted previously, the Merger Agreement determined aggregate consideration paid based on the Average Closing Price. For GAAP purposes, the purchase price is determined based on the closing price of the Common Stock as of the TransRe Acquisition Date.

[2] The $816.0 million cash consideration was fixed as of the date of the Merger Agreement, predicated on the 57.4 million shares of common stock of Old TransRe outstanding at that date multiplied by $14.22 per share. Therefore, the additional 0.2 million of Old TransRe shares outstanding as of the TransRe Acquisition Date do not cause the cash consideration to increase and are adjusted for in this presentation.