Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Jun. 30, 2013 Security	Sep. 30, 2012 Security
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Jun. 30, 2013
Available-for-sale securities, Fair Value, Less than 12 months	$ 4,436.9	$ 681.1
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(185.8)	(11.9)
Available-for-sale securities, Fair Value, 12 months or longer	725.1	323.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	31.7	18.7
Available-for-sale securities, Fair Value, Total	5,162.0	1,004.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(217.5)	(30.6)
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months	537	100
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer		56
Total number of available-for-sale securities in an unrealized loss position		156
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	300.6	169.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(4.1)	(1.0)
Available-for-sale securities, Fair Value, 12 months or longer	77.6	7.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	0.5	0.6
Available-for-sale securities, Fair Value, Total	378.2	177.3
Available-for-sale Securities, Gross Unrealized Losses, Total	(4.6)	(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	28.7	0.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.3)	(0.8)
Available-for-sale securities, Fair Value, 12 months or longer	0	10.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	0.4	1.6
Available-for-sale securities, Fair Value, Total	28.7	11.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.7)	(2.4)
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	3,133.5	411.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(142.4)	(8.1)
Available-for-sale securities, Fair Value, 12 months or longer	396.0	45.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	21.1	1.9
Available-for-sale securities, Fair Value, Total	3,529.5	456.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(163.5)	(10.0)
Equities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	90.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(3.2)
Available-for-sale securities, Fair Value, 12 months or longer	33.9	44.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	1.4	1.3
Available-for-sale securities, Fair Value, Total	124.8	44.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(4.6)	(1.3)
Hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	75.4	13.4
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	2.5	(0.4)
Available-for-sale securities, Fair Value, 12 months or longer	40.7	107.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.6)	9.2
Available-for-sale securities, Fair Value, Total	116.1	121.1
Available-for-sale Securities, Gross Unrealized Losses, Total	3.1	(9.6)
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	350.9	71.1
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(21.4)	(1.1)
Available-for-sale securities, Fair Value, 12 months or longer	101.4
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	6.0
Available-for-sale securities, Fair Value, Total	452.3	71.1
Available-for-sale Securities, Gross Unrealized Losses, Total	(27.4)	(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	2.5	1.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	0	(0.2)
Available-for-sale securities, Fair Value, 12 months or longer	6.3	6.1
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	0.4	0.1
Available-for-sale securities, Fair Value, Total	8.8	7.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.4)	(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	255.5	12.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(8.0)	(0.3)
Available-for-sale securities, Fair Value, 12 months or longer	69.2	101.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	1.3	4.0
Available-for-sale securities, Fair Value, Total	324.7	114.7
Available-for-sale Securities, Gross Unrealized Losses, Total	(9.3)	(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	198.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(3.9)
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	198.9
Available-for-sale Securities, Gross Unrealized Losses, Total	$ (3.9)