Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013 Security	Sep. 30, 2012 Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	$ 1,425.8	$ 681.1
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(15.1)	(11.9)
Available-for-sale securities, Fair Value, 12 months or longer	251.9	323.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(4.8)	(18.7)
Available-for-sale securities, Fair Value, Total	1,677.7	1,004.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(19.9)	(30.6)
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months	213	100
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer	51	56
Total number of available-for-sale securities in an unrealized loss position	264	156
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	116.3	169.8
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(0.5)	(1.0)
Available-for-sale securities, Fair Value, 12 months or longer	21.8	7.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.2)	(0.6)
Available-for-sale securities, Fair Value, Total	138.1	177.3
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.7)	(1.6)
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months		0.8
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months		(0.8)
Available-for-sale securities, Fair Value, 12 months or longer	0.2	10.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.5)	(1.6)
Available-for-sale securities, Fair Value, Total	0.2	11.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.5)	(2.4)
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	1,044.4	411.3
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(11.2)	(8.1)
Available-for-sale securities, Fair Value, 12 months or longer	125.3	45.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.8)	(1.9)
Available-for-sale securities, Fair Value, Total	1,169.7	456.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(13.0)	(10.0)
Equities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	17.7
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(0.7)
Available-for-sale securities, Fair Value, 12 months or longer	32.2	44.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.2)	(1.3)
Available-for-sale securities, Fair Value, Total	49.9	44.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(1.9)	(1.3)
Hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months		13.4
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months		(0.4)
Available-for-sale securities, Fair Value, 12 months or longer		107.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer		(9.2)
Available-for-sale securities, Fair Value, Total		121.1
Available-for-sale Securities, Gross Unrealized Losses, Total		(9.6)
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	79.1	71.1
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(0.8)	(1.1)
Available-for-sale securities, Fair Value, 12 months or longer
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer
Available-for-sale securities, Fair Value, Total	79.1	71.1
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.8)	(1.1)
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	4.9	1.8
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(0.2)	(0.2)
Available-for-sale securities, Fair Value, 12 months or longer	4.1	6.1
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.1)	(0.1)
Available-for-sale securities, Fair Value, Total	9.0	7.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.3)	(0.3)
Non-agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	143.4	12.9
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(1.6)	(0.3)
Available-for-sale securities, Fair Value, 12 months or longer	68.3	101.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.0)	(4.0)
Available-for-sale securities, Fair Value, Total	211.7	114.7
Available-for-sale Securities, Gross Unrealized Losses, Total	(2.6)	(4.3)
U.S. Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	20.0
Available-for-sale Securities, Gross Unrealized Losses, Less than 12 Months	(0.1)
Available-for-sale securities, Fair Value, 12 months or longer	0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	0
Available-for-sale securities, Fair Value, Total	20.0
Available-for-sale Securities, Gross Unrealized Losses, Total	$ (0.1)