Investments - Available-for-Sale Securities (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Sep. 30, 2014		Sep. 30, 2013
Schedule of Investments [Line Items]
Document Period End Date	Dec. 31, 2014
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 1,160.8		$ 709.5		$ 1,066.0		$ 523.5
Total Gains (Losses) Included in Earnings	0.5		0.6
Total investments, Carrying Value	19,740.3				19,252.5
Available-for-sale Debt Securities, Amortized Cost Basis	16,939.8
Total investments, Cost or Amortized Cost	18,890.3				18,409.8
Investments, Unrealized Losses	(240.3)				(158.4)
Investments, Unrealized Gains	1,090.3				1,001.1
Total investment, Estimated Fair Value	19,740.3				19,252.5
Total Gains (Losses) Included in AOCI	11.1		(6.7)
Purchases	92.2		192.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	3.1		0.5
Net transfer In (Out) of Level 3 (a)	(5.9)	[1]
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	34.6		9.8		37.0		5.0
Total Gains (Losses) Included in Earnings	0
Total Gains (Losses) Included in AOCI	0		(0.2)
Purchases	3.5		5.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0
Net transfer In (Out) of Level 3 (a)	(5.9)	[1]			(5.9)	[1]
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	120.4		6.0		83.1		5.7
Total Gains (Losses) Included in AOCI	0.8		0.3
Purchases	36.8
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.3
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	907.6		607.1		850.0		461.1
Total Gains (Losses) Included in Earnings	(0.1)
Total Gains (Losses) Included in AOCI	8.4		(6.1)
Purchases	51.9		152.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	2.6		0.5
Hybrids [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			34.3
Total Gains (Losses) Included in AOCI			(0.7)
Purchases			35.0
Municipals [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	38.8				37.2
Total Gains (Losses) Included in Earnings	0
Total Gains (Losses) Included in AOCI	1.8
Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0.2
Net transfer In (Out) of Level 3 (a)	0
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
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	15,555.0				15,009.3
Equity Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	6.1		10.8		6.0		10.7
Total Gains (Losses) Included in Earnings			0.1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	55.7				31.2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(58.5)				(50.0)
Available-for-sale Securities, Equity Securities	717.4				768.1
Total investments, Carrying Value	717.4				768.1
Available-for-sale Equity Securities, Amortized Cost Basis	720.2				786.9
Total Gains (Losses) Included in AOCI	0.1		0
Purchases	0		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]
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	25.3				23.0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(6.8)				(5.1)
Available-for-sale Securities, Equity Securities	625.9				663.6
Total investments, Carrying Value	625.9				663.6
Available-for-sale Equity Securities, Amortized Cost Basis	607.4				645.7
Equity Securities [Member] | Held for trading [Member]
Schedule of Investments [Line Items]
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	30.4				8.2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(51.7)				(44.9)
Available-for-sale Securities, Equity Securities	91.5				104.5
Total investments, Carrying Value	91.5				104.5
Available-for-sale Equity Securities, Amortized Cost Basis	112.8				141.2
Fixed-maturity securities, available-for-sale [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	910.6				846.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(172.6)				(103.7)
Available-for-sale Securities, Debt Securities	17,677.8				17,211.5
Total investments, Carrying Value	17,677.8				17,211.5
Available-for-sale Debt Securities, Amortized Cost Basis	16,939.8				16,468.7
Fixed-maturity securities, available-for-sale [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	8.3				10.9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(32.1)				(18.8)
Available-for-sale Securities, Debt Securities	1,839.1				1,792.9
Total investments, Carrying Value	1,839.1				1,792.9
Available-for-sale Debt Securities, Amortized Cost Basis	1,862.9				1,800.8
Fixed-maturity securities, available-for-sale [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	20.0				21.3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1.0)				(2.0)
Available-for-sale Securities, Debt Securities	668.4				636.9
Total investments, Carrying Value	668.4				636.9
Available-for-sale Debt Securities, Amortized Cost Basis	649.4				617.6
Fixed-maturity securities, available-for-sale [Member] | Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	550.6				499.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(95.0)				(48.9)
Available-for-sale Securities, Debt Securities	9,721.4				9,795.8
Total investments, Carrying Value	9,721.4				9,795.8
Available-for-sale Debt Securities, Amortized Cost Basis	9,265.8				9,345.5
Fixed-maturity securities, available-for-sale [Member] | Hybrids [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	55.6				52.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(22.8)				(15.2)
Available-for-sale Securities, Debt Securities	1,301.5				1,316.1
Total investments, Carrying Value	1,301.5				1,316.1
Available-for-sale Debt Securities, Amortized Cost Basis	1,268.7				1,279.1
Fixed-maturity securities, available-for-sale [Member] | Municipals [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	143.2				116.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(2.8)				(6.3)
Available-for-sale Securities, Debt Securities	1,361.9				1,259.8
Total investments, Carrying Value	1,361.9				1,259.8
Available-for-sale Debt Securities, Amortized Cost Basis	1,221.5				1,149.9
Fixed-maturity securities, available-for-sale [Member] | Agency residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	124.6				140.3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(18.1)				(11.1)
Available-for-sale Securities, Debt Securities	2,201.9				2,114.0
Total investments, Carrying Value	2,201.9				2,114.0
Available-for-sale Debt Securities, Amortized Cost Basis	2,095.4				1,984.8
Fixed-maturity securities, available-for-sale [Member] | U.S. Government securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	8.3				6.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(0.8)				(1.4)
Available-for-sale Securities, Debt Securities	583.6				296.0
Total investments, Carrying Value	583.6				296.0
Available-for-sale Debt Securities, Amortized Cost Basis	576.1				291.0
Derivative Financial Instruments, Assets [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	191.4				177.7
Trading Securities	306.2				296.3
Other Investments Gross Unrealized Gain	124.0				123.3
Other Short Term Investments Gross Unrealized Losses	(9.2)				(4.7)
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	791.9				811.6
Trading Securities	791.9				811.6
Other Invested Assets [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost					164.9
Trading Securities					165.0
Other Investments Gross Unrealized Gain					0.1
Other Invested Assets [Member] | Other Investments Asset [Member] | All Other Securities [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	247.0
Trading Securities	247.0
Other Investments Gross Unrealized Gain	0
Other Short Term Investments Gross Unrealized Losses	$ 0

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