Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2014 Securities Security	Sep. 30, 2013 Security
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Mar. 31, 2014
Available-for-sale securities, Fair Value, Less than 12 months	$ 3,355.0	$ 5,405.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(92.0)	(235.9)
Available-for-sale securities, Fair Value, 12 months or longer	1,171.4	520.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(46.4)	(28.0)
Available-for-sale securities, Fair Value, Total	4,526.4	5,925.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(138.4)	(263.9)
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months	440	588
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer	156	78
Total number of available-for-sale securities in an unrealized loss position	596	666
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	483.4	329.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(6.1)	(4.5)
Available-for-sale securities, Fair Value, 12 months or longer	120.3	81.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(2.4)	(0.7)
Available-for-sale securities, Fair Value, Total	603.7	410.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(8.5)	(5.2)
Commercial mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	46.8	26.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.6)	(0.5)
Available-for-sale securities, Fair Value, 12 months or longer	0.2	4.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.3)	(1.1)
Available-for-sale securities, Fair Value, Total	47.0	31.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(1.9)	(1.6)
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	1,902.2	3,457.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(54.9)	(175.0)
Available-for-sale securities, Fair Value, 12 months or longer	769.9	186.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(31.7)	(10.1)
Available-for-sale securities, Fair Value, Total	2,672.1	3,643.2
Available-for-sale Securities, Gross Unrealized Losses, Total	(86.6)	(185.1)
Equities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	70.0	118.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(6.4)	(9.1)
Available-for-sale securities, Fair Value, 12 months or longer	26.0	32.2
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.7)	(1.2)
Available-for-sale securities, Fair Value, Total	96.0	150.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(8.1)	(10.3)
Hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	19.1	52.0
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.4)	(3.3)
Available-for-sale securities, Fair Value, 12 months or longer	10.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.2)
Available-for-sale securities, Fair Value, Total	29.6	52.0
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.6)	(3.3)
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	359.8	333.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(13.5)	(27.3)
Available-for-sale securities, Fair Value, 12 months or longer	107.5	144.4
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(6.3)	(13.5)
Available-for-sale securities, Fair Value, Total	467.3	477.7
Available-for-sale Securities, Gross Unrealized Losses, Total	(19.8)	(40.8)
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	9.9	9.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses		(0.1)
Available-for-sale securities, Fair Value, 12 months or longer	0.6	1.1
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer		(0.2)
Available-for-sale securities, Fair Value, Total	10.5	10.9
Available-for-sale Securities, Gross Unrealized Losses, Total		(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	335.9	325.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(7.6)	(12.2)
Available-for-sale securities, Fair Value, 12 months or longer	96.7	69.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(2.5)	(1.2)
Available-for-sale securities, Fair Value, Total	432.6	395.1
Available-for-sale Securities, Gross Unrealized Losses, Total	(10.1)	(13.4)
U.S. Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	127.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(2.5)
Available-for-sale securities, Fair Value, 12 months or longer	39.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.3)
Available-for-sale securities, Fair Value, Total	167.6
Available-for-sale Securities, Gross Unrealized Losses, Total	(2.8)
US Government Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months		753.9
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
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer
Available-for-sale securities, Fair Value, Total		753.9
Available-for-sale Securities, Gross Unrealized Losses, Total		$ (3.9)