Goodwill and Intangible Asset, Net of Accumulated Amortization Expense on Consolidated Balance Sheets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	9 Months Ended
	Jul. 20, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment	Sep. 30, 2012 Reinsurance Segment	Mar. 06, 2012 Reinsurance Segment Gain contingency on dispute previously in arbitration	Apr. 26, 2012 Bourn and Koch, Incorporated
Goodwill and Intangible Assets Disclosure [Line Items]
Acquisition of business, purchase price					$ 55.0
Acquisition of business, estimated contingent consideration					8.0
Finite lived intangible assets				36.0
Settlement payment awarded	75.0
Pre-tax net benefit resulting from legal settlement		$ 23.5	$ 23.5