Investments - Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Sep. 30, 2013	Sep. 30, 2012
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2013
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 709.5		$ 313.5		$ 523.5	$ 206.0
Total Gains (Losses) Included in Earnings	0.6		(0.2)
Asset Backed Securities Fair Value Disclosure	794.0				560.4
Total investments, Carrying Value	17,668.3				16,465.9
Derivative investments, Cost or Amortized Cost	148.3				141.7
Derivative investments, Gross Unrealized Gains	147.0				88.5
Derivative investments, Gross Unrealized Losses	0.8				8.4
Derivative investments, Estimated Fair Value	294.5				221.8
Derivative investments, Carrying Value	294.5				221.8
Asset-Backed Loans, Cost or Amortized Cost	794.0				560.4
Asset-Backed Loans, Gross Unrealized Gain
Other Short Term Investments Gross Unrealized Losses
Asset-Backed Loans, at Carrying Value	794.0				560.4
Total investments, Cost or Amortized Cost	17,341.2				16,190.1
Investments, Unrealized Losses	(322.3)				(311.5)
Investments, Unrealized Gains	649.4				587.3
Total investment, Gross Unrealized Losses	(280.1)				(263.9)
Total investment, Estimated Fair Value	17,668.3				16,465.9
Total Gains (Losses) Included in AOCI	(6.7)		(2.1)
Purchases	192.6		134.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			9.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.5		0.7
Net transfer In (Out) of Level 3 (a)		[1]	(14.1)	[2]
Asset-backed securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	9.8		5.3		5.0	15.9
Total Gains (Losses) Included in Earnings	0
Total investment, Gross Unrealized Losses	(8.0)				(5.2)
Total Gains (Losses) Included in AOCI	(0.2)		(0.1)
Purchases
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Net transfer In (Out) of Level 3 (a)		[1]	(10.5)	[2]
Commercial mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6.0		6.1		5.7	5.0
Total Gains (Losses) Included in Earnings
Total investment, Gross Unrealized Losses	(3.2)				(1.6)
Total Gains (Losses) Included in AOCI	0.3		0.1
Purchases			1.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Net transfer In (Out) of Level 3 (a)		[1]		[2]
Corporates [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	607.1		256.1		461.1	135.3
Total Gains (Losses) Included in Earnings	0		(0.2)
Total investment, Gross Unrealized Losses	(189.3)				(185.1)
Total Gains (Losses) Included in AOCI	(6.1)		(2.0)
Purchases	152.6		133.2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		9.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.5		0.7
Net transfer In (Out) of Level 3 (a)		[1]	0.1	[2]
Hybrids [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			5.0			8.8
Total Gains (Losses) Included in Earnings
Total investment, Gross Unrealized Losses	(3.2)				(3.3)
Total Gains (Losses) Included in AOCI			(0.1)
Purchases
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements
Net transfer In (Out) of Level 3 (a)			(3.7)	[2]
Municipals [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	34.3
Total Gains (Losses) Included in Earnings	0
Total investment, Gross Unrealized Losses	(45.0)				(40.8)
Total Gains (Losses) Included in AOCI	(0.7)
Purchases	35.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Net transfer In (Out) of Level 3 (a)		[1]
Agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Total investment, Gross Unrealized Losses	(0.1)				(0.3)
Non-agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Unrealized gains included in AOCI related to the non-credit portion of other than-temporary impairments on non-agency residential-mortgage-backed securities	0.9
Total investment, Gross Unrealized Losses	(11.3)				(13.4)
Unrealized losses included in AOCI related to the non-credit portion of other than-temporary impairments on non-agency residential-mortgage-backed securities	1.9				(1.9)
Other Marketable Securities, Current	13,727.8				19.4
U.S. Government securities [Member]
Schedule of Investments [Line Items]
Total investment, Gross Unrealized Losses	(6.2)
Equity Securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	418.3				394.7
Equity Securities, Estimated Fair Value	375.9				352.5
Total investments, Carrying Value	375.9				352.5
Total investment, Gross Unrealized Gains	12.8				7.3
Total investment, Gross Unrealized Losses	55.2				49.5
Equity Securities [Member] | Available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	293.9				274.6
Equity Securities, Estimated Fair Value	286.9				271.0
Total investments, Carrying Value	286.9				271.0
Total investment, Gross Unrealized Gains	6.8				6.7
Total investment, Gross Unrealized Losses	13.8				10.3
Equity Securities [Member] | Held for trading [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	124.4				120.1
Equity Securities, Estimated Fair Value	89.0				81.5
Total investments, Carrying Value	89.0				81.5
Total investment, Gross Unrealized Gains	6.0				0.6
Total investment, Gross Unrealized Losses	41.4				39.2
Fixed-maturity securities, available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	15,863.3				15,062.1
Equity Securities, Estimated Fair Value	16,086.6				15,300.0
Total investments, Carrying Value	16,086.6				15,300.0
Total investment, Gross Unrealized Gains	489.6				491.5
Total investment, Gross Unrealized Losses	266.3				253.6
Fixed-maturity securities, available-for-sale [Member] | Asset-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,590.7				1,505.7
Equity Securities, Estimated Fair Value	1,601.8				1,523.1
Total investments, Carrying Value	1,601.8				1,523.1
Total investment, Gross Unrealized Gains	19.1				22.6
Total investment, Gross Unrealized Losses	8.0				5.2
Fixed-maturity securities, available-for-sale [Member] | Commercial mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	422.6				431.3
Equity Securities, Estimated Fair Value	444.6				454.4
Total investments, Carrying Value	444.6				454.4
Total investment, Gross Unrealized Gains	25.2				24.7
Total investment, Gross Unrealized Losses	3.2				1.6
Fixed-maturity securities, available-for-sale [Member] | Corporates [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	9,963.8				9,314.7
Equity Securities, Estimated Fair Value	10,046.6				9,418.3
Total investments, Carrying Value	10,046.6				9,418.3
Total investment, Gross Unrealized Gains	272.1				288.7
Total investment, Gross Unrealized Losses	189.3				185.1
Fixed-maturity securities, available-for-sale [Member] | Hybrids [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	400.5				412.6
Equity Securities, Estimated Fair Value	418.1				428.8
Total investments, Carrying Value	418.1				428.8
Total investment, Gross Unrealized Gains	20.8				19.5
Total investment, Gross Unrealized Losses	3.2				3.3
Fixed-maturity securities, available-for-sale [Member] | Municipals [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,121.2				998.8
Equity Securities, Estimated Fair Value	1,121.4				1,007.0
Total investments, Carrying Value	1,121.4				1,007.0
Total investment, Gross Unrealized Gains	45.2				49.0
Total investment, Gross Unrealized Losses	45.0				40.8
Fixed-maturity securities, available-for-sale [Member] | Agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	88.9				96.5
Equity Securities, Estimated Fair Value	91.0				98.6
Total investments, Carrying Value	91.0				98.6
Total investment, Gross Unrealized Gains	2.2				2.4
Total investment, Gross Unrealized Losses	0.1				0.3
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	1,564.1				1,304.0
Equity Securities, Estimated Fair Value	1,652.0				1,368.0
Total investments, Carrying Value	1,652.0				1,368.0
Total investment, Gross Unrealized Gains	99.2				77.4
Total investment, Gross Unrealized Losses	11.3				13.4
Fixed-maturity securities, available-for-sale [Member] | U.S. Government securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	711.5				998.5
Equity Securities, Estimated Fair Value	711.1				1,001.8
Total investments, Carrying Value	711.1				1,001.8
Total investment, Gross Unrealized Gains	5.8				7.2
Total investment, Gross Unrealized Losses	6.2				3.9
Other invested assets [Member] | Policy loans and other invested assets [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	117.3				31.2
Equity Securities, Estimated Fair Value	117.3				31.2
Total investments, Carrying Value	117.3				31.2
Total investment, Gross Unrealized Gains	0
Total investment, Gross Unrealized Losses	$ 0

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[2] (a)The net transfers in and out of Level 3 during the three months ended December 30, 2012 was exclusively to or from Level 2.