Fair Value of Goodwill and Identifiable Intangible Assets and Respective Useful Lives (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
Schedule of Goodwill and Purchased Intangible Assets [Line Items]
Intangible asset, fair value	$ 87.7
Goodwill	6.0
Total goodwill and intangibles	93.7
Estimated useful life	Indefinite
Other intangibles, Amortization Method	Straight line
Lloyd's Syndicate Capacity
Schedule of Goodwill and Purchased Intangible Assets [Line Items]
Intangible asset, fair value	78.7
Estimated useful life	Indefinite
Other Intangibles
Schedule of Goodwill and Purchased Intangible Assets [Line Items]
Intangible asset, fair value	$ 9.0
Other Intangibles | Minimum
Schedule of Goodwill and Purchased Intangible Assets [Line Items]
Intangible asset, Estimated Useful Life	2 years
Other Intangibles | Maximum
Schedule of Goodwill and Purchased Intangible Assets [Line Items]
Intangible asset, Estimated Useful Life	5 years