Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014 Security	Sep. 30, 2013 Security
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Sep. 30, 2014
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 108.8	$ 263.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	2,577.1	5,405.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	69.4	28.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,862.6	5,925.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	39.4	235.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	2,285.5	520.0
Total fixed maturity available-for-sale securities, Amortized Cost	16,468.7
Total fixed maturity available-for-sale securities, Fair Value	17,211.5
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	319	588
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	310	78
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	629	666
U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1.4	3.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	37.3	753.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	119.0	753.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1	3.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	81.7
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	18.8	5.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	825.8	329.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	7.0	0.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,114.0	410.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	11.8	4.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	288.2	81.5
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2.0	1.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	160.3	26.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.1	1.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	160.7	31.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.9	0.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.4	4.9
Other securities which provide for periodic payments, Amortized Cost	617.6
Other securities which provide for periodic payments, Fair Value	636.9
Structured hybrids
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	474.5
Other securities which provide for periodic payments, Fair Value	472.0
Agency Residential Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.1	0.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	24.1	9.8
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	24.7	10.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1	0.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0.6	1.1
Other securities which provide for periodic payments, Amortized Cost	104.3
Other securities which provide for periodic payments, Fair Value	107.3
Non-agency residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	11.0	13.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	274.4	325.2
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5.3	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	451.4	395.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5.7	12.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	177.0	69.9
Other securities which provide for periodic payments, Amortized Cost	1,880.5
Other securities which provide for periodic payments, Fair Value	2,006.7
Corporates
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	48.9	185.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	816.6	3,457.2
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	32.6	10.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,944.4	3,643.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	16.3	175.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,127.8	186.0
Equities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5.1	10.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	180.4	118.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2.9	1.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235.3	150.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.2	9.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	54.9	32.2
Hybrids
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	15.2	3.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	258.2	52.0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	12.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	548.2	52.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.3	3.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	290.0
Municipals
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6.3	40.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		333.3
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	6.3	13.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	264.9	477.7
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		27.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	264.9	144.4
Non-structured Hybrids | Corporate | Municipal | U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	370.0
Due after one year through five years, Amortized Cost	2,297.6
Due after five years through ten years, Amortized Cost	3,128.9
Due after ten years, Amortized Cost	5,794.5
Subtotal, Amortized Cost	11,591.0
Due in one year or less, Fair Value	372.8
Due after one year through five years, Fair Value	2,360.2
Due after five years through ten years, Fair Value	3,232.7
Due after ten years, Fair Value	6,230.0
Subtotal, Fair Value	12,195.7
Fixed-maturity securities, available-for-sale
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	16,468.7	15,062.1
Available-for-sale Securities, Debt Securities	17,211.5	15,300.0
Fixed-maturity securities, available-for-sale | U.S. Government
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	291.0	998.5
Available-for-sale Securities, Debt Securities	296.0	1,001.8
Fixed-maturity securities, available-for-sale | Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	1,800.8	1,505.7
Available-for-sale Securities, Debt Securities	1,792.9	1,523.1
Fixed-maturity securities, available-for-sale | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	617.6	431.3
Available-for-sale Securities, Debt Securities	636.9	454.4
Fixed-maturity securities, available-for-sale | Agency Residential Mortgage Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	104.3	96.5
Available-for-sale Securities, Debt Securities	107.3	98.6
Fixed-maturity securities, available-for-sale | Non-agency residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	1,880.5	1,304.0
Available-for-sale Securities, Debt Securities	2,006.7	1,368.0
Fixed-maturity securities, available-for-sale | Corporates
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	9,345.5	9,314.7
Available-for-sale Securities, Debt Securities	9,795.8	9,418.3
Fixed-maturity securities, available-for-sale | Hybrids
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	1,279.1	412.6
Available-for-sale Securities, Debt Securities	1,316.1	428.8
Fixed-maturity securities, available-for-sale | Municipals
Schedule of Available-for-sale Securities [Line Items]
Total fixed maturity available-for-sale securities, Amortized Cost	1,149.9	998.8
Available-for-sale Securities, Debt Securities	$ 1,259.8	$ 1,007.0