Investments - Consolidated Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 17,391.5
Available For Sale Securities Carrying Value	19,110.7	$ 19,252.5
Unrealized Amortized Cost On Securities	19,087.2	18,409.8
Investments, Unrealized Gains	671.7	1,001.1
Investments, Unrealized Losses	647.8	158.4
Equity Securities
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value	649.4	768.1
Available-for-sale Equity Securities, Amortized Cost Basis	612.1	786.9
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	41.3	31.2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	4.0	50.0
Equity Securities \| Available-for-sale
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value	616.6	663.6
Available-for-sale Equity Securities, Amortized Cost Basis	593.4	645.7
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	27.2	23.0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	4.0	5.1
Equity Securities \| Held for trading
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value		104.5
Available-for-sale Equity Securities, Amortized Cost Basis	18.7	141.2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	14.1	8.2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax		44.9
Fixed-maturity securities, available-for-sale
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	17,391.5	16,468.7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	617.6	846.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	494.3	103.7
Available For Sale Securities Carrying Value	17,514.8	17,211.5
Fixed-maturity securities, available-for-sale \| Asset-backed securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,902.6	1,800.8
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	5.0	10.9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	47.1	18.8
Available For Sale Securities Carrying Value	1,860.5	1,792.9
Fixed-maturity securities, available-for-sale \| Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	877.9	617.6
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	13.7	21.3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	9.4	2.0
Available For Sale Securities Carrying Value	882.2	636.9
Fixed-maturity securities, available-for-sale \| Corporates
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	9,547.7	9,345.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	351.1	499.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	354.1	48.9
Available For Sale Securities Carrying Value	9,544.7	9,795.8
Fixed-maturity securities, available-for-sale \| Hybrids
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,210.8	1,279.1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	44.8	52.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	42.3	15.2
Available For Sale Securities Carrying Value	1,213.3	1,316.1
Fixed-maturity securities, available-for-sale \| Municipals
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,519.7	1,149.9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	103.0	116.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	15.1	6.3
Available For Sale Securities Carrying Value	1,607.6	1,259.8
Fixed-maturity securities, available-for-sale \| Agency Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,099.4	1,984.8
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	89.4	140.3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	26.3	11.1
Available For Sale Securities Carrying Value	2,162.5	2,114.0
Fixed-maturity securities, available-for-sale \| U.S. Government
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	233.4	291.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	10.6	6.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		1.4
Available For Sale Securities Carrying Value	244.0	296.0
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	335.8	811.6
Trading Securities	335.8	811.6
Commercial Mortgage Loan [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	489.2	136.2
Other Investments Gross Unrealized Gain	(0.4)
Trading Securities	489.2	136.2
Derivative Financial Instruments, Assets [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	219.0	177.7
Other Investments Gross Unrealized Gain	(12.4)	(123.3)
Other Short Term Investments Gross Unrealized Losses	149.5	4.7
Trading Securities	81.9	296.3
Other invested assets \| Other invested assets \| Policy loans and other invested assets
Schedule of Investments [Line Items]
Trading Securities, Cost	39.6	28.7
Other Investments Gross Unrealized Gain		0.1
Trading Securities	39.6	28.8
Estimate of Fair Value Measurement [Member]
Schedule of Investments [Line Items]
Available For Sale Securities Fair Value	19,111.1	19,252.5
Estimate of Fair Value Measurement [Member] \| Equity Securities
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	649.4	768.1
Estimate of Fair Value Measurement [Member] \| Equity Securities \| Available-for-sale
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	616.6	663.6
Estimate of Fair Value Measurement [Member] \| Equity Securities \| Held for trading
Schedule of Investments [Line Items]
Available-for-sale Securities, Equity Securities	32.8	104.5
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	17,514.8	17,211.5
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Asset-backed securities
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	1,860.5	1,792.9
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	882.2	636.9
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Corporates
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	9,544.7	9,795.8
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Hybrids
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	1,213.3	1,316.1
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Municipals
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	1,607.6	1,259.8
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| Agency Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	2,162.5	2,114.0
Estimate of Fair Value Measurement [Member] \| Fixed-maturity securities, available-for-sale \| U.S. Government
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	244.0	296.0
Estimate of Fair Value Measurement [Member] \| Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Trading Securities	335.8	811.6
Estimate of Fair Value Measurement [Member] \| Commercial Mortgage Loan [Member]
Schedule of Investments [Line Items]
Trading Securities	489.6	136.2
Estimate of Fair Value Measurement [Member] \| Derivative Financial Instruments, Assets [Member]
Schedule of Investments [Line Items]
Trading Securities	81.9	296.3
Estimate of Fair Value Measurement [Member] \| Other invested assets \| Other invested assets \| Policy loans and other invested assets
Schedule of Investments [Line Items]
Trading Securities	$ 39.6	$ 28.8

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