Net Intangible Assets (Liabilities) Acquired in Merger with Transatlantic (Detail) (USD $)						1 Months Ended								1 Months Ended
	Sep. 30, 2012	Dec. 31, 2011		Sep. 30, 2012 Transatlantic	Mar. 06, 2012 Transatlantic	Mar. 06, 2012 Transatlantic State and foreign insurance licenses	Mar. 06, 2012 Transatlantic Trade name	Mar. 06, 2012 Transatlantic Value of business in-force	Mar. 06, 2012 Transatlantic Loss and LAE reserves	Mar. 06, 2012 Transatlantic Renewal rights	Mar. 06, 2012 Transatlantic Leases	Mar. 06, 2012 Transatlantic Gain contingency on dispute previously in arbitration	Sep. 30, 2012 Transatlantic Gain contingency on dispute previously in arbitration	Mar. 06, 2012 Transatlantic Internally-developed software
Schedule of Intangible Assets Disclosure [Line Items]
Economic useful lives of finite lived intangible assets								1 year	15 years	14 years	10 years			2 years 6 months
Economic useful lives of indefinite lived intangible assets						Indefinite	Indefinite
Economic useful lives of finite lived intangible assets, description												As settled
Finite lived intangible assets								$ 291,400,000	$ (98,800,000)	$ 44,000,000	$ (28,100,000)	$ 36,000,000		$ 10,000,000
Indefinite lived intangible assets						19,000,000	50,000,000
Net intangible assets, before amortization, as of the Acquisition Date					323,500,000
Amortization (from the Acquisition Date through September 30, 2012)	(267,700,000)	(54,300,000)	[1]	(210,800,000)
Settlement of gain contingency on dispute previously in arbitration													(36,000,000)
Net intangible assets, net of amortization	$ 165,669,000	$ 90,863,000		$ 76,700,000

[1] See Note 4 of Notes to Consolidated Financial Statements set forth in Item 8 of the 2011 10-K for additional detail on goodwill and other intangible assets.