Investments - Net Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Gain (Loss) on Investments [Abstract]
Document Period End Date			Mar. 31, 2014
Net realized gains before other-than-temporary impairments	$ 7.9	$ 73.0	$ 17.7	$ 245.5
Gross other-than-temporary impairments		(0.4)		(0.9)
Net realized gains on fixed maturity available-for-sale securities	7.9	72.6	17.7	244.6
Realized gains on equity securities	5.4	1.9	10.8	1.9
Net realized gains on securities	13.3	74.5	28.5	246.5
Realized gains (losses) on certain derivative instruments	50.0	29.7	110.6	45.3
Unrealized gains on certain derivative instruments	(27.5)	102.5	39.3	61.3
Change in fair value of derivatives	22.5	132.2	149.9	106.6
Realized gains on other invested assets	5.1		4.4	0.1
Net investment gains (losses)	$ 40.9	$ 206.7	$ 182.8	$ 353.2