Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2013
Gross investment income	$ 204.8	$ 182.1
External investment expense	(3.6)	(4.1)
Net investment income	201.2	178.0
Fixed maturity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	191.8	167.6
Equity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	4.5	4.7
Policy loans [Member]
Schedule of Investments [Line Items]
Gross investment income	0.2	0.3
Invested cash and short-term investments [Member]
Schedule of Investments [Line Items]
Gross investment income	0.1	0.8
Other investments [Member]
Schedule of Investments [Line Items]
Gross investment income	$ 8.2	$ 8.7