Investments - Net Investment Income (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
Schedule of Investments [Line Items]
Document Period End Date			Jun. 30, 2014
Gross investment income	$ 217.7	$ 191.8	$ 632.8	$ 548.0
External investment expense	(6.8)	(3.6)	(14.3)	(10.5)
Net investment income	210.9	188.2	618.5	537.5
Fixed maturity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	196.6	178.0	581.9	506.4
Equity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	7.1	3.9	16.7	11.5
Policy loans [Member]
Schedule of Investments [Line Items]
Gross investment income	0.2	0.1	0.5	0.6
Invested cash and short-term investments [Member]
Schedule of Investments [Line Items]
Gross investment income	0.1	0.2	0.2	1.4
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Gross investment income	12.2	8.2	30.7	25.8
Other investments [Member]
Schedule of Investments [Line Items]
Gross investment income	$ 1.5	$ 1.4	$ 2.8	$ 2.3