Basis of Presentation (Schedule of Effect of Retrospective Adoption on Individual Financial Statement Line Items in Consolidated Balance Sheet) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Basis Of Presentation [Line Items]
Reinsurance recoverables	$ 1,016.5	$ 1,144.1
Deferred acquisition costs, net	107.7	101.5
Deferred tax liabilities, net	49.7	18.5
Retained earnings	783.7	736.0
As Previously Reported [Member]
Basis Of Presentation [Line Items]
Reinsurance recoverables		1,143.5
Deferred acquisition costs, net		125.7
Deferred tax liabilities, net		26.1
Retained earnings		752.0
As Adjusted [Member]
Basis Of Presentation [Line Items]
Reinsurance recoverables		1,144.1
Deferred acquisition costs, net		101.5
Deferred tax liabilities, net		18.5
Retained earnings		736.0
Effect of Change [Member]
Basis Of Presentation [Line Items]
Reinsurance recoverables		0.6
Deferred acquisition costs, net		(24.2)
Deferred tax liabilities, net		(7.6)
Retained earnings		$ (16.0)