Investments - Net Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Investments, Debt and Equity Securities [Abstract]
Net realized gains before other-than-temporary impairments	$ 332.9	$ 287.2	$ 34.9
Gross other-than-temporary impairments	(2.9)	(24.3)	(17.5)
Non-credit portion of other-than-temporary impairments included in other comprehensive income		1.5	(0.5)
Net realized gains on fixed maturity available-for-sale securities	330.0	264.4	16.9
Realized gains on equity securities	12.6	0.9	(10.9)
Net realized gains on securities	342.6	265.3	6.0
Realized gains (losses) on certain derivative instruments	20.5	(10.3)	(44.8)
Unrealized gains on certain derivative instruments	148.6	156.3	(126.0)
Change in fair value of derivatives	169.1	146.0	(170.8)
Realized gains on other invested assets	(0.1)	(1.3)	(2.1)
Net investment gains (losses)	$ 511.6	$ 410.0	$ (166.9)