Unaudited Pro Forma Consolidated Information (Detail) (Transatlantic, USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011
Transatlantic
Business Acquisition, Pro Forma Information [Line Items]
Total revenues	$ 3,802.9	[1]	$ 3,953.8	[1]
Net earnings (losses)	$ 358.1	[1],[2]	$ (10.8)	[1],[2]
Basic earnings per share	$ 21.16		$ (0.62)

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

[2] Among other adjustments, reflects adjustments to amortization expense on the Transatlantic Senior Notes resulting from the adjustment to their fair value as of the Pro Forma Acquisition Dates. Such adjustment reduced the interest expense of Transatlantic.