Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013
Schedule of Investments [Line Items]
Document Period End Date			Mar. 31, 2014
Gross investment income	$ 210.3	$ 174.1	$ 415.1	$ 356.2
External investment expense	(3.9)	(2.8)	(7.5)	(6.9)
Net investment income	206.4	171.3	407.6	349.3
Fixed maturity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	193.5	163.0	385.3	328.4
Equity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	5.1	2.9	9.6	7.6
Policy loans [Member]
Schedule of Investments [Line Items]
Gross investment income	0.1	0.2	0.3	0.5
Invested cash and short-term investments [Member]
Schedule of Investments [Line Items]
Gross investment income		0.4	0.1	1.2
Asset Backed Loans [Member]
Schedule of Investments [Line Items]
Gross investment income	11.1	7.0	18.5	17.6
Other investments [Member]
Schedule of Investments [Line Items]
Gross investment income	$ 0.5	$ 0.6	$ 1.3	$ 0.9