Investments - Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]
Equity Securities, Gross Unrealized Losses	$ (19.9)	$ (30.6)
Total investments, Carrying Value	17,033.5	16,918.4
Derivative investments, Cost or Amortized Cost	141.8	142.1
Derivative investments, Gross Unrealized Gains	124.6	67.0
Derivative investments, Gross Unrealized Losses	(3.8)	(8.4)
Derivative investments, Estimated Fair Value	262.6	200.7
Derivative investments, Carrying Value	262.6	200.7
Asset-Backed Loans, Cost or Amortized Cost	241.6	180.1
Asset-Backed Loans, Gross Unrealized Gain
Asset-Backed Loans, Gross Unrealized Losses
Asset-Backed Loans, Estimated Fair Value	241.6	180.1
Asset-Backed Loans, at Carrying Value	241.6	180.1
Total investments, Cost or Amortized Cost	15,965.3	15,846.3
Total investment, Gross Unrealized Gains	1,136.3	1,156.1
Total investment, Gross Unrealized Losses	(68.1)	(84.0)
Total investment, Estimated Fair Value	17,033.5	16,918.4
Equity securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	349.5	429.3
Equity Securities, Gross Unrealized Gains	10.2	11.9
Equity Securities, Gross Unrealized Losses	(46.3)	(46.3)
Equity Securities, Estimated Fair Value	313.4	394.9
Total investments, Carrying Value	313.4	394.9
Equity securities [Member] | Available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	247.4	237.5
Equity Securities, Gross Unrealized Gains	10.2	11.9
Equity Securities, Gross Unrealized Losses	(1.9)	(1.3)
Equity Securities, Estimated Fair Value	255.7	248.1
Total investments, Carrying Value	255.7	248.1
Equity securities [Member] | Held for trading [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	102.1	191.8
Equity Securities, Gross Unrealized Gains
Equity Securities, Gross Unrealized Losses	(44.4)	(45.0)
Equity Securities, Estimated Fair Value	57.7	146.8
Total investments, Carrying Value	57.7	146.8
Fixed-maturity securities, available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	15,200.0	15,041.0
Equity Securities, Gross Unrealized Gains	1,001.5	1,077.2
Equity Securities, Gross Unrealized Losses	(18.0)	(29.3)
Equity Securities, Estimated Fair Value	16,183.5	16,088.9
Total investments, Carrying Value	16,183.5	16,088.9
Fixed-maturity securities, available-for-sale [Member] | Asset-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,388.5	1,010.9
Equity Securities, Gross Unrealized Gains	30.6	18.6
Equity Securities, Gross Unrealized Losses	(0.7)	(1.6)
Equity Securities, Estimated Fair Value	1,418.4	1,027.9
Total investments, Carrying Value	1,418.4	1,027.9
Fixed-maturity securities, available-for-sale [Member] | Commercial mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	506.8	520.0
Equity Securities, Gross Unrealized Gains	39.9	36.2
Equity Securities, Gross Unrealized Losses	(0.5)	(2.4)
Equity Securities, Estimated Fair Value	546.2	553.8
Total investments, Carrying Value	546.2	553.8
Fixed-maturity securities, available-for-sale [Member] | Corporates [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	10,118.0	10,211.8
Equity Securities, Gross Unrealized Gains	686.3	807.2
Equity Securities, Gross Unrealized Losses	(13.0)	(10.0)
Equity Securities, Estimated Fair Value	10,791.3	11,009.0
Total investments, Carrying Value	10,791.3	11,009.0
Fixed-maturity securities, available-for-sale [Member] | Hybrids [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	438.8	519.0
Equity Securities, Gross Unrealized Gains	28.1	18.8
Equity Securities, Gross Unrealized Losses		(9.6)
Equity Securities, Estimated Fair Value	466.9	528.2
Total investments, Carrying Value	466.9	528.2
Fixed-maturity securities, available-for-sale [Member] | Municipals [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	969.1	1,083.2
Equity Securities, Gross Unrealized Gains	109.4	141.9
Equity Securities, Gross Unrealized Losses	(0.8)	(1.1)
Equity Securities, Estimated Fair Value	1,077.7	1,224.0
Total investments, Carrying Value	1,077.7	1,224.0
Fixed-maturity securities, available-for-sale [Member] | Agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	122.2	149.5
Equity Securities, Gross Unrealized Gains	4.1	5.8
Equity Securities, Gross Unrealized Losses	(0.3)	(0.3)
Equity Securities, Estimated Fair Value	126.0	155.0
Total investments, Carrying Value	126.0	155.0
Fixed-maturity securities, available-for-sale [Member] | Non-agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,209.5	629.1
Equity Securities, Gross Unrealized Gains	91.6	35.8
Equity Securities, Gross Unrealized Losses	(2.6)	(4.3)
Equity Securities, Estimated Fair Value	1,298.5	660.6
Total investments, Carrying Value	1,298.5	660.6
Fixed-maturity securities, available-for-sale [Member] | U.S. Government securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	447.1	917.5
Equity Securities, Gross Unrealized Gains	11.5	12.9
Equity Securities, Gross Unrealized Losses	(0.1)
Equity Securities, Estimated Fair Value	458.5	930.4
Total investments, Carrying Value	458.5	930.4
Other invested assets [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost		53.8
Equity Securities, Gross Unrealized Gains
Equity Securities, Gross Unrealized Losses
Equity Securities, Estimated Fair Value		53.8
Total investments, Carrying Value		53.8
Other invested assets [Member] | U.S. Treasuries and certificate of deposit, held-to-maturity [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost		35.0
Equity Securities, Gross Unrealized Gains
Equity Securities, Gross Unrealized Losses
Equity Securities, Estimated Fair Value		35.0
Total investments, Carrying Value		35.0
Other invested assets [Member] | Policy loans and other invested assets [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	32.4	18.8
Equity Securities, Gross Unrealized Gains
Equity Securities, Gross Unrealized Losses
Equity Securities, Estimated Fair Value	32.4	18.8
Total investments, Carrying Value	$ 32.4	$ 18.8