Segments of Business - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended			3 Months Ended	9 Months Ended	3 Months Ended	7 Months Ended	3 Months Ended	7 Months Ended				3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Segment	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment Foreign	Sep. 30, 2012 Reinsurance Segment Foreign	Sep. 30, 2012 Reinsurance Segment Foreign United Kingdom	Sep. 30, 2012 Reinsurance Segment Foreign United Kingdom	Sep. 30, 2012 Reinsurance Segment Broker One Concentration Risk	Sep. 30, 2012 Reinsurance Segment Broker Two Concentration Risk	Sep. 30, 2012 Reinsurance Segment Broker Three Concentration Risk	Sep. 30, 2012 Insurance Segment	Sep. 30, 2011 Insurance Segment	Sep. 30, 2012 Insurance Segment	Sep. 30, 2011 Insurance Segment	Sep. 30, 2012 Corporate activities		Sep. 30, 2012 Corporate activities
Segment Reporting Information [Line Items]
Number of reportable segments			2
Gross premiums written	$ 1,176,400,000	$ 270,400,000	$ 3,078,800,000	$ 880,600,000		$ 886,500,000	$ 2,096,900,000	$ 444,200,000	$ 1,046,600,000						$ 295,800,000	$ 270,400,000	$ 994,800,000	$ 880,600,000	$ (5,900,000)	[1]	$ (12,900,000)	[1]
Net premiums earned	1,092,775,000	190,209,000	2,622,914,000	555,067,000		881,100,000	2,017,500,000	421,800,000	989,100,000	165,100,000	381,900,000				211,700,000	190,200,000	605,400,000	555,100,000
Identifiable assets	22,757,642,000		22,757,642,000		6,478,089,000	16,500,000,000	16,500,000,000								5,300,000,000		5,300,000,000		1,000,000,000		1,000,000,000
Cash and invested assets						14,400,000,000	14,400,000,000								3,800,000,000		3,800,000,000		800,000,000		800,000,000
Equity	$ 6,581,069,000		$ 6,581,069,000		$ 2,925,677,000	$ 4,400,000,000	$ 4,400,000,000								$ 2,100,000,000		$ 2,100,000,000		$ 100,000,000		$ 100,000,000
Percentage of gross premiums written												34.00%	26.00%	17.00%

[1] Includes elimination of minor reinsurance activity between segments.