Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013 Security	Sep. 30, 2013 Security
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Dec. 31, 2013
Available-for-sale securities, Fair Value, Less than 12 months	$ 5,293.9	$ 5,405.9
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(232.5)	(235.9)
Available-for-sale securities, Fair Value, 12 months or longer	850.3	520.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(47.6)	(28.0)
Available-for-sale securities, Fair Value, Total	6,144.2	5,925.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(280.1)	(263.9)
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months	634	588
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer	109	78
Total number of available-for-sale securities in an unrealized loss position	743	666
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	488.5	329.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(6.8)	(4.5)
Available-for-sale securities, Fair Value, 12 months or longer	127.5	81.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.2)	(0.7)
Available-for-sale securities, Fair Value, Total	616.0	410.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(8.0)	(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	29.4	26.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.5)	(0.5)
Available-for-sale securities, Fair Value, 12 months or longer	5.9	4.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(2.7)	(1.1)
Available-for-sale securities, Fair Value, Total	35.3	31.5
Available-for-sale Securities, Gross Unrealized Losses, Total	(3.2)	(1.6)
Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	3,355.4	3,457.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(163.0)	(175.0)
Available-for-sale securities, Fair Value, 12 months or longer	477.9	186.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(26.3)	(10.1)
Available-for-sale securities, Fair Value, Total	3,833.3	3,643.2
Available-for-sale Securities, Gross Unrealized Losses, Total	(189.3)	(185.1)
Equities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	131.7	118.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(13.8)	(9.1)
Available-for-sale securities, Fair Value, 12 months or longer	6.9	32.2
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer		(1.2)
Available-for-sale securities, Fair Value, Total	138.6	150.8
Available-for-sale Securities, Gross Unrealized Losses, Total	(13.8)	(10.3)
Hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	106.3	52.0
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(3.1)	(3.3)
Available-for-sale securities, Fair Value, 12 months or longer	8.3
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.1)
Available-for-sale securities, Fair Value, Total	114.6	52.0
Available-for-sale Securities, Gross Unrealized Losses, Total	(3.2)	(3.3)
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	478.2	333.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(28.4)	(27.3)
Available-for-sale securities, Fair Value, 12 months or longer	168.1	144.4
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(16.6)	(13.5)
Available-for-sale securities, Fair Value, Total	646.3	477.7
Available-for-sale Securities, Gross Unrealized Losses, Total	(45.0)	(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	11.9	9.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.1)	(0.1)
Available-for-sale securities, Fair Value, 12 months or longer	0.5	1.1
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	0	(0.2)
Available-for-sale securities, Fair Value, Total	12.4	10.9
Available-for-sale Securities, Gross Unrealized Losses, Total	(0.1)	(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	354.5	325.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(10.6)	(12.2)
Available-for-sale securities, Fair Value, 12 months or longer	55.2	69.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.7)	(1.2)
Available-for-sale securities, Fair Value, Total	409.7	395.1
Available-for-sale Securities, Gross Unrealized Losses, Total	(11.3)	(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	338.0
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(6.2)
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	338.0
Available-for-sale Securities, Gross Unrealized Losses, Total	(6.2)
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)