Investments - Consolidated Investments (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]
Gross Unrealized Losses	$ (263.9)	$ (30.6)
Carrying Value	16,465.9	16,918.4
Derivative investments, Cost or Amortized Cost	141.7	142.1
Derivative investments, Gross Unrealized Gains	88.5	67.0
Derivative investments, Gross Unrealized Losses	8.4	8.4
Derivative investments, Estimated Fair Value	221.8	200.7
Derivative investments, Carrying Value	221.8	200.7
Asset-Backed Loans, Cost or Amortized Cost	560.4	180.1
Asset-Backed Loans, Gross Unrealized Gain
Asset-Backed Loans, Gross Unrealized Losses
Asset Backed Securities Fair Value Disclosure	560.4	180.1
Asset-Backed Loans, at Carrying Value	560.4	180.1
Total investments, Cost or Amortized Cost	16,190.1	15,846.3
Total Investments, Gross Unrealized Losses	(311.5)	(84.0)
Total Investments, Gross Unrealized Gains	587.3	1,156.1
Total investment, Estimated Fair Value	16,465.9	16,918.4
Asset-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Losses	(5.2)	(1.6)
Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Losses	(1.6)	(2.4)
Corporates
Schedule of Investments [Line Items]
Gross Unrealized Losses	(185.1)	(10.0)
Hybrids
Schedule of Investments [Line Items]
Gross Unrealized Losses	(3.3)	(9.6)
Municipals
Schedule of Investments [Line Items]
Gross Unrealized Losses	(40.8)	(1.1)
Agency Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Gross Unrealized Losses	(0.3)	(0.3)
Non-agency residential mortgage-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Losses	(13.4)	(4.3)
U.S. Government
Schedule of Investments [Line Items]
Gross Unrealized Losses	(3.9)
Equity Securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	394.7	429.3
Gross Unrealized Gains	7.3	11.9
Gross Unrealized Losses	49.5	46.3
Estimated Fair Value	352.5	394.9
Carrying Value	352.5	394.9
Equity Securities | Available-for-sale
Schedule of Investments [Line Items]
Cost or Amortized Cost	274.6	237.5
Gross Unrealized Gains	6.7	11.9
Gross Unrealized Losses	10.3	1.3
Estimated Fair Value	271.0	248.1
Carrying Value	271.0	248.1
Equity Securities | Held for trading
Schedule of Investments [Line Items]
Cost or Amortized Cost	120.1	191.8
Gross Unrealized Gains	0.6
Gross Unrealized Losses	39.2	45.0
Estimated Fair Value	81.5	146.8
Carrying Value	81.5	146.8
Fixed-maturity securities, available-for-sale
Schedule of Investments [Line Items]
Cost or Amortized Cost	15,062.1	15,041.0
Gross Unrealized Gains	491.5	1,077.2
Gross Unrealized Losses	253.6	29.3
Estimated Fair Value	15,300.0	16,088.9
Carrying Value	15,300.0	16,088.9
Fixed-maturity securities, available-for-sale | Asset-backed securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	1,505.7	1,010.9
Gross Unrealized Gains	22.6	18.6
Gross Unrealized Losses	5.2	1.6
Estimated Fair Value	1,523.1	1,027.9
Carrying Value	1,523.1	1,027.9
Fixed-maturity securities, available-for-sale | Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	431.3	520.0
Gross Unrealized Gains	24.7	36.2
Gross Unrealized Losses	1.6	2.4
Estimated Fair Value	454.4	553.8
Carrying Value	454.4	553.8
Fixed-maturity securities, available-for-sale | Corporates
Schedule of Investments [Line Items]
Cost or Amortized Cost	9,314.7	10,211.8
Gross Unrealized Gains	288.7	807.2
Gross Unrealized Losses	185.1	10.0
Estimated Fair Value	9,418.3	11,009.0
Carrying Value	9,418.3	11,009.0
Fixed-maturity securities, available-for-sale | Hybrids
Schedule of Investments [Line Items]
Cost or Amortized Cost	412.6	519.0
Gross Unrealized Gains	19.5	18.8
Gross Unrealized Losses	3.3	9.6
Estimated Fair Value	428.8	528.2
Carrying Value	428.8	528.2
Fixed-maturity securities, available-for-sale | Municipals
Schedule of Investments [Line Items]
Cost or Amortized Cost	998.8	1,083.2
Gross Unrealized Gains	49.0	141.9
Gross Unrealized Losses	40.8	1.1
Estimated Fair Value	1,007.0	1,224.0
Carrying Value	1,007.0	1,224.0
Fixed-maturity securities, available-for-sale | Agency Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	96.5	149.5
Gross Unrealized Gains	2.4	5.8
Gross Unrealized Losses	0.3	0.3
Estimated Fair Value	98.6	155.0
Carrying Value	98.6	155.0
Fixed-maturity securities, available-for-sale | Non-agency residential mortgage-backed securities
Schedule of Investments [Line Items]
Cost or Amortized Cost	1,304.0	629.1
Gross Unrealized Gains	77.4	35.8
Gross Unrealized Losses	13.4	4.3
Estimated Fair Value	1,368.0	660.6
Carrying Value	1,368.0	660.6
Fixed-maturity securities, available-for-sale | U.S. Government
Schedule of Investments [Line Items]
Cost or Amortized Cost	998.5	917.5
Gross Unrealized Gains	7.2	12.9
Gross Unrealized Losses	3.9
Estimated Fair Value	1,001.8	930.4
Carrying Value	1,001.8	930.4
Other invested assets
Schedule of Investments [Line Items]
Cost or Amortized Cost		53.8
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value		53.8
Carrying Value		53.8
Other invested assets | U.S. Treasuries and certificate of deposit, held-to-maturity
Schedule of Investments [Line Items]
Cost or Amortized Cost		35.0
Gross Unrealized Gains
Gross Unrealized Losses
Estimated Fair Value		35.0
Carrying Value		35.0
Other invested assets | Policy loans and other invested assets
Schedule of Investments [Line Items]
Cost or Amortized Cost	31.2	18.8
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	31.2	18.8
Carrying Value	$ 31.2	$ 18.8