Investments - Fair Value and Gross Unrealized Losses of Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015 Security	Sep. 30, 2014 Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	$ 2,193.9	$ 2,577.1
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(60.0)	(39.4)
Available-for-sale securities, Fair Value, 12 months or longer	1,911.0	2,285.5
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(80.1)	(69.4)
Available-for-sale securities, Fair Value, Total	4,104.9	4,862.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	140.1	108.8
Total number of available-for-sale securities in an unrealized loss position, Less than 12 months	322	319
Total number of available-for-sale securities in an unrealized loss position, 12 months or longer	245	310
Total number of available-for-sale securities in an unrealized loss position	567	629
Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	560.1	825.8
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(7.7)	(11.8)
Available-for-sale securities, Fair Value, 12 months or longer	474.5	288.2
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(10.8)	(7.0)
Available-for-sale securities, Fair Value, Total	1,034.6	1,114.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	18.5	18.8
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	51.3	160.3
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.4)	(0.9)
Available-for-sale securities, Fair Value, 12 months or longer	8.4	0.4
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.4)	(1.1)
Available-for-sale securities, Fair Value, Total	59.7	160.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.8	2.0
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	853.5	816.6
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(32.6)	(16.3)
Available-for-sale securities, Fair Value, 12 months or longer	761.2	1,127.8
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(42.0)	(32.6)
Available-for-sale securities, Fair Value, Total	1,614.7	1,944.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	74.6	48.9
Equities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	35.0	180.4
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.7)	(2.2)
Available-for-sale securities, Fair Value, 12 months or longer	50.2	54.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(2.2)	(2.9)
Available-for-sale securities, Fair Value, Total	85.2	235.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2.9	5.1
Hybrids [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	149.2	258.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(7.8)	(2.3)
Available-for-sale securities, Fair Value, 12 months or longer	143.8	290.0
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(11.8)	(12.9)
Available-for-sale securities, Fair Value, Total	293.0	548.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	19.6	15.2
Municipals [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	63.2
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(0.6)
Available-for-sale securities, Fair Value, 12 months or longer	125.8	264.9
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(1.2)	(6.3)
Available-for-sale securities, Fair Value, Total	189.0	264.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1.8	6.3
Agency residential mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	411.7	298.5
Available For Sale Securities In A Continuous Unrealized Loss Position Less Than 12 Months Aggregate Losses	(10.2)	(5.8)
Available-for-sale securities, Fair Value, 12 months or longer	297.5	177.6
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(11.6)	(5.3)
Available-for-sale securities, Fair Value, Total	709.2	476.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	21.8	11.1
U.S. Government securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Fair Value, Less than 12 months	69.9	37.3
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	49.6	81.7
Available-for-sale Securities, Gross Unrealized Losses, 12 months or longer	(0.1)	(1.3)
Available-for-sale securities, Fair Value, Total	119.5	119.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0.1	$ 1.4

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

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