Investments (Schedule Of Amortized Cost And Fair Values Of Fixed Maturity Investments, By Contractual Maturity) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Amortized Cost, Due in one year or less	$ 240.1
Amortized Cost, Due after one year through five years	1,164.7
Amortized Cost, Due after five years through ten years	763.6
Amortized Cost, Thereafter	191.7
Amortized Cost, Structured securities	625.4
Amortized Cost, Total	2,985.5
Fair Value, Due in one year or less	242.7
Fair Value, Due after one year through five years	1,209.8
Fair Value, Due after five years through ten years	837.1
Fair Value, Thereafter	209.5
Fair Value, Structured securities	658.4
Fair Value, Total	$ 3,157.5