Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]
Gross investment income	$ 859.8	$ 751.5	$ 734.4
External investment expense	(17.1)	(16.8)	(11.7)
Net investment income	842.2	734.7	722.7
Fixed maturity available-for-sale securities
Schedule of Investments [Line Items]
Gross investment income	784.8	686.2	707.1
Equity available-for-sale securities
Schedule of Investments [Line Items]
Gross investment income	24.7	14.8	14.0
Policy loans
Schedule of Investments [Line Items]
Gross investment income	0.7	0.8	0.7
Invested cash and short-term investments
Schedule of Investments [Line Items]
Gross investment income	0.3	1.4	4.9
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Gross investment income	41.5	35.4	8.6
Other investments
Schedule of Investments [Line Items]
Gross investment income	$ 7.1	$ 12.9	$ (0.9)