Investments - Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2014		Mar. 31, 2013		Mar. 31, 2014		Mar. 31, 2013		Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule of Investments [Line Items]
Document Period End Date					Mar. 31, 2014
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 755.6		$ 419.0		$ 755.6		$ 419.0		$ 709.5	$ 523.5	$ 313.5	$ 206.0
Total Gains (Losses) Included in Earnings			(0.1)		0.6		(0.3)
Asset Backed Securities Fair Value Disclosure	795.7				795.7					560.4
Total investments, Carrying Value	18,312.8				18,312.8					16,465.9
Derivative investments, Cost or Amortized Cost	154.6				154.6					141.7
Derivative investments, Gross Unrealized Gains	120.7				120.7					88.5
Derivative investments, Gross Unrealized Losses	2.3				2.3					8.4
Derivative investments, Estimated Fair Value	273.0				273.0					221.8
Derivative investments, Carrying Value	273.0				273.0					221.8
Asset-Backed Loans, Cost or Amortized Cost	795.7				795.7					560.4
Asset-Backed Loans, Gross Unrealized Gain
Other Short Term Investments Gross Unrealized Losses
Asset-Backed Loans, at Carrying Value	795.7				795.7					560.4
Total investments, Cost or Amortized Cost	17,610.1				17,610.1					16,190.1
Investments, Unrealized Losses	(174.6)				(174.6)					(311.5)
Investments, Unrealized Gains	877.3				877.3					587.3
Total investment, Gross Unrealized Losses	(138.4)				(138.4)					(263.9)
Total investment, Estimated Fair Value	18,312.8				18,312.8					16,465.9
Total Gains (Losses) Included in AOCI	11.3		3.3		4.6		1.2
Purchases	41.7		154.2		234.3		288.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales							9.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	1.9		13.0		2.4		13.7
Net transfer In (Out) of Level 3 (a)	(5.0)	[1]	(38.9)	[2]	(5.0)	[1]	(53.0)	[2]
Asset-backed securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	10.7		5.3		10.7		5.3		9.8	5.0	5.3	15.9
Total Gains (Losses) Included in Earnings	0				0
Total investment, Gross Unrealized Losses	(8.5)				(8.5)					(5.2)
Total Gains (Losses) Included in AOCI	(0.1)				(0.3)		(0.1)
Purchases	0				5.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0				0
Net transfer In (Out) of Level 3 (a)	1.0	[1]		[2]	1.0	[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			6.2				6.2		6.0	5.7	6.1	5.0
Total Gains (Losses) Included in Earnings
Total investment, Gross Unrealized Losses	(1.9)				(1.9)					(1.6)
Total Gains (Losses) Included in AOCI			0.1		0.3		0.2
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				0
Net transfer In (Out) of Level 3 (a)	(6.0)	[1]		[2]	(6.0)	[1]		[2]
Corporates [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	657.0		356.5		657.0		356.5		607.1	461.1	256.1	135.3
Total Gains (Losses) Included in Earnings	0		(0.1)		0		(0.3)
Total investment, Gross Unrealized Losses	(86.6)				(86.6)					(185.1)
Total Gains (Losses) Included in AOCI	10.1		3.2		4.0		1.2
Purchases	41.7		144.2		194.3		277.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0				0		9.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	1.9		13.0		2.4		13.7
Net transfer In (Out) of Level 3 (a)		[1]	(33.9)	[2]		[1]	(33.8)	[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	(0.6)				(0.6)					(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)			(5.0)	[2]			(8.7)	[2]
Municipals [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	35.6				35.6				34.3
Total Gains (Losses) Included in Earnings	0				0
Total investment, Gross Unrealized Losses	(19.8)				(19.8)					(40.8)
Total Gains (Losses) Included in AOCI	1.3				0.6
Purchases	0				35.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0				0
Net transfer In (Out) of Level 3 (a)		[1]				[1]
Agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Total investment, Gross Unrealized Losses										(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				0.9
Total investment, Gross Unrealized Losses	(10.1)				(10.1)					(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)
Other Marketable Securities, Current	14,334.4				14,334.4					19.4
U.S. Government securities [Member]
Schedule of Investments [Line Items]
Total investment, Gross Unrealized Losses	(2.8)				(2.8)
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			10.0				10.0				0	0
Total Gains (Losses) Included in Earnings	0		0				0
Equity Securities, Cost or Amortized Cost	486.8				486.8					394.7
Equity Securities, Estimated Fair Value	484.9				484.9					352.5
Total investments, Carrying Value	484.9				484.9					352.5
Total investment, Gross Unrealized Gains	40.1				40.1					7.3
Total investment, Gross Unrealized Losses	42.0				42.0					49.5
Total Gains (Losses) Included in AOCI			0				0
Purchases			10.0				10.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales			0				0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements			0				0
Net transfer In (Out) of Level 3 (a)			0	[1]			0	[1]
Equity Securities [Member] | Available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	346.6				346.6					274.6
Equity Securities, Estimated Fair Value	353.1				353.1					271.0
Total investments, Carrying Value	353.1				353.1					271.0
Total investment, Gross Unrealized Gains	14.6				14.6					6.7
Total investment, Gross Unrealized Losses	8.1				8.1					10.3
Equity Securities [Member] | Held for trading [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	140.2				140.2					120.1
Equity Securities, Estimated Fair Value	131.8				131.8					81.5
Total investments, Carrying Value	131.8				131.8					81.5
Total investment, Gross Unrealized Gains	25.5				25.5					0.6
Total investment, Gross Unrealized Losses	33.9				33.9					39.2
Fixed-maturity securities, available-for-sale [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	16,040.5				16,040.5					15,062.1
Equity Securities, Estimated Fair Value	16,626.6				16,626.6					15,300.0
Total investments, Carrying Value	16,626.6				16,626.6					15,300.0
Total investment, Gross Unrealized Gains	716.4				716.4					491.5
Total investment, Gross Unrealized Losses	130.3				130.3					253.6
Fixed-maturity securities, available-for-sale [Member] | Asset-backed securities [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,576.6				1,576.6					1,505.7
Equity Securities, Estimated Fair Value	1,587.2				1,587.2					1,523.1
Total investments, Carrying Value	1,587.2				1,587.2					1,523.1
Total investment, Gross Unrealized Gains	19.1				19.1					22.6
Total investment, Gross Unrealized Losses	8.5				8.5					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	463.3				463.3					431.3
Equity Securities, Estimated Fair Value	486.6				486.6					454.4
Total investments, Carrying Value	486.6				486.6					454.4
Total investment, Gross Unrealized Gains	25.2				25.2					24.7
Total investment, Gross Unrealized Losses	1.9				1.9					1.6
Fixed-maturity securities, available-for-sale [Member] | Corporates [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	10,177.3				10,177.3					9,314.7
Equity Securities, Estimated Fair Value	10,522.1				10,522.1					9,418.3
Total investments, Carrying Value	10,522.1				10,522.1					9,418.3
Total investment, Gross Unrealized Gains	431.4				431.4					288.7
Total investment, Gross Unrealized Losses	86.6				86.6					185.1
Fixed-maturity securities, available-for-sale [Member] | Hybrids [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	423.6				423.6					412.6
Equity Securities, Estimated Fair Value	451.6				451.6					428.8
Total investments, Carrying Value	451.6				451.6					428.8
Total investment, Gross Unrealized Gains	28.6				28.6					19.5
Total investment, Gross Unrealized Losses	0.6				0.6					3.3
Fixed-maturity securities, available-for-sale [Member] | Municipals [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	1,182.8				1,182.8					998.8
Equity Securities, Estimated Fair Value	1,240.0				1,240.0					1,007.0
Total investments, Carrying Value	1,240.0				1,240.0					1,007.0
Total investment, Gross Unrealized Gains	77.0				77.0					49.0
Total investment, Gross Unrealized Losses	19.8				19.8					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	96.9				96.9					96.5
Equity Securities, Estimated Fair Value	99.4				99.4					98.6
Total investments, Carrying Value	99.4				99.4					98.6
Total investment, Gross Unrealized Gains	2.5				2.5					2.4
Total investment, Gross Unrealized Losses										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	1,730.9				1,730.9					1,304.0
Equity Securities, Estimated Fair Value	1,847.1				1,847.1					1,368.0
Total investments, Carrying Value	1,847.1				1,847.1					1,368.0
Total investment, Gross Unrealized Gains	126.3				126.3					77.4
Total investment, Gross Unrealized Losses	10.1				10.1					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	389.1				389.1					998.5
Equity Securities, Estimated Fair Value	392.6				392.6					1,001.8
Total investments, Carrying Value	392.6				392.6					1,001.8
Total investment, Gross Unrealized Gains	6.3				6.3					7.2
Total investment, Gross Unrealized Losses	2.8				2.8					3.9
Other invested assets [Member] | Policy loans and other invested assets [Member]
Schedule of Investments [Line Items]
Equity Securities, Cost or Amortized Cost	132.5				132.5					31.2
Equity Securities, Estimated Fair Value	132.6				132.6					31.2
Total investments, Carrying Value	132.6				132.6					31.2
Total investment, Gross Unrealized Gains	0.1				0.1
Total investment, Gross Unrealized Losses	$ 0				$ 0

[1] This includes a $6.0 transfer to asset-backed securities from commercial mortgage-backed securities, the remaining transfers were from level 3 to level 2.

[2] (a)The net transfers in and out of Level 3 during the six months ended March 31, 2013 were exclusively to or from Level 2.