Investments - Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Apr. 01, 2012	Mar. 31, 2013	Apr. 01, 2012
Schedule of Investments [Line Items]
Gross investment income	$ 174.8	$ 176.2	$ 357.0	$ 366.1
External investment expense	(2.8)	(3.2)	(6.9)	(6.3)
Net investment income	172.0	173.0	350.1	359.8
Fixed maturity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	165.9	170.5	333.5	357.7
Equity available-for-sale securities [Member]
Schedule of Investments [Line Items]
Gross investment income	2.9	3.4	7.6	6.0
Policy loans [Member]
Schedule of Investments [Line Items]
Gross investment income	0.2	0.1	0.5	0.4
Invested cash and short-term investments [Member]
Schedule of Investments [Line Items]
Gross investment income	0.4	1.3	1.2	1.4
Other investments [Member]
Schedule of Investments [Line Items]
Gross investment income	$ 5.4	$ 0.9	$ 14.2	$ 0.6