Investments - Net Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Gain (Loss) on Investments [Abstract]
Document Period End Date			Jun. 30, 2014
Net realized gains before other-than-temporary impairments	$ 75.0	$ 34.7	$ 92.7	$ 280.2
Gross other-than-temporary impairments	(0.6)	(0.7)	(0.6)	(1.6)
Net realized gains on fixed maturity available-for-sale securities	74.4	34.0	92.1	278.6
Realized gains on equity securities	3.0	4.5	13.8	6.4
Net realized gains on securities	77.4	38.5	105.9	285.0
Realized gains (losses) on certain derivative instruments	62.7	54.0	173.3	99.3
Unrealized gains on certain derivative instruments	38.9	(34.0)	78.2	27.3
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	0.3	0	0.3	0
Change in fair value of derivatives	101.9	20.0	251.8	126.6
Realized gains on other invested assets	5.3	(0.2)	9.7	(0.1)
Net investment gains (losses)	$ 184.6	$ 58.3	$ 367.4	$ 411.5