Investments - Net Investment Gains (Losses) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Gain (Loss) on Investments [Line Items]
Net realized gains before other-than-temporary impairments	$ 101.9	$ 332.9	$ 287.2
Gross other-than-temporary impairments	(0.6)	(2.9)	(24.3)
Non-credit portion of other-than-temporary impairments included in other comprehensive income			1.5
Net realized gains on fixed maturity available-for-sale securities	101.3	330.0	264.4
Realized gains on equity securities	13.5	12.6	0.9
Net realized gains on securities	114.8	342.6	265.3
Realized gains (losses) on certain derivative instruments	37.7	148.6	(10.3)
Unrealized gains on certain derivative instruments	233.8	20.5	156.3
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	(0.1)	0	0
Change in fair value of derivatives	271.4	169.1	146.0
Realized gains (losses) on other invested assets	9.1	(0.1)	(1.3)
Net investment gains	395.3	511.6	410.0
Net Investment Income
Gain (Loss) on Investments [Line Items]
Available For Sale Securities On Fixed Maturity Under Commitment Agreement	5,033.4	8,986.9	4,603.0
Available-for-sale Securities, Gross Realized Gains	108.5	351.2	295.9
Available-for-sale Securities, Gross Realized Losses	$ (4.9)	$ 18.3	$ 13.8