Net Intangible Assets (Liabilities) Acquired in Merger with Transatlantic (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended					0 Months Ended
	Dec. 31, 2012 Value of business in-force	Dec. 31, 2012 Loss and LAE reserves	Dec. 31, 2012 Leases	Dec. 31, 2012 Internally-developed software	Mar. 06, 2012 TransRe	Mar. 06, 2012 TransRe State and foreign insurance licenses	Mar. 06, 2012 TransRe Trade name	Mar. 06, 2012 TransRe Value of business in-force	Mar. 06, 2012 TransRe Loss and LAE reserves	Mar. 06, 2012 TransRe Renewal rights	Mar. 06, 2012 TransRe Leases	Mar. 06, 2012 TransRe Gain contingency on dispute previously in arbitration	Mar. 06, 2012 TransRe Internally-developed software
Schedule of Intangible Assets Disclosure [Line Items]
Finite lived intangible assets								$ 291.4	$ (98.8)	$ 44.0	$ (28.1)	$ 36.0	$ 10.0
Indefinite lived intangible assets						19.0	50.0
Net intangible assets, before amortization, as of the TransRe Acquisition Date					$ 323.5
Economic useful lives of finite lived intangible assets	1 year	15 years	10 years	2 years 6 months				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