Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule of Investments [Line Items]
Gross investment income	$ 235.3	$ 204.8
External investment expense	(4.5)	(3.6)
Net investment income	230.8	201.2
Fixed maturity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	212.8	191.8
Equity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	9.9	4.5
Policy loans [Member]
Schedule of Investments [Line Items]
Gross investment income	0.2	0.2
Invested cash and short-term investments [Member]
Schedule of Investments [Line Items]
Gross investment income	0	0.1
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Gross investment income	9.3	7.4
Other investments [Member]
Schedule of Investments [Line Items]
Gross investment income	$ 3.1	$ 0.8