Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Schedule of Investments [Line Items]
Gross investment income	$ 751.5	$ 734.4	$ 376.9
External investment expense	(16.8)	(11.7)	(7.1)
Net investment income	734.7	722.7	369.8
Fixed maturity available-for-sale securities
Schedule of Investments [Line Items]
Gross investment income		707.1	364.8
Equity available-for-sale securities
Schedule of Investments [Line Items]
Gross investment income	14.8	14.0	10.2
Policy loans
Schedule of Investments [Line Items]
Gross investment income	0.8	0.7	1.5
Invested cash and short-term investments
Schedule of Investments [Line Items]
Gross investment income	1.4	4.9	0.1
Other investments
Schedule of Investments [Line Items]
Gross investment income	$ 48.3	$ 7.7	$ 0.3