Investments - Consolidated Investments (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Schedule of Investments [Line Items]
Document Period End Date	Sep. 30, 2014
Available-for-sale Debt Securities, Amortized Cost Basis	$ 16,468.7
Available For Sale Securities Carrying Value	19,252.5	16,465.9
Unrealized Amortized Cost On Securities	18,409.8	16,190.1
Investments, Unrealized Gains	1,001.1	587.3
Investments, Unrealized Losses	(158.4)	(311.5)
Available For Sale Securities Fair Value	19,252.5	16,465.9
Equity Securities
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value	768.1	352.5
Available-for-sale Equity Securities, Amortized Cost Basis	786.9	394.7
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	31.2	7.3
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(50.0)	(49.5)
Available-for-sale Securities, Equity Securities	768.1	352.5
Equity Securities | Available-for-sale
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value	663.6	271.0
Available-for-sale Equity Securities, Amortized Cost Basis	645.7	274.6
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	23.0	6.7
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(5.1)	(10.3)
Available-for-sale Securities, Equity Securities	663.6	271.0
Equity Securities | Held for trading
Schedule of Investments [Line Items]
Available For Sale Securities Carrying Value	104.5	81.5
Available-for-sale Equity Securities, Amortized Cost Basis	141.2	120.1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	8.2	0.6
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	(44.9)	(39.2)
Available-for-sale Securities, Equity Securities	104.5	81.5
Fixed-maturity securities, available-for-sale
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	16,468.7	15,062.1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	846.5	491.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(103.7)	(253.6)
Available-for-sale Securities, Debt Securities	17,211.5	15,300.0
Available For Sale Securities Carrying Value	17,211.5	15,300.0
Fixed-maturity securities, available-for-sale | Asset-backed securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,800.8	1,505.7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	10.9	22.6
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(18.8)	(5.2)
Available-for-sale Securities, Debt Securities	1,792.9	1,523.1
Available For Sale Securities Carrying Value	1,792.9	1,523.1
Fixed-maturity securities, available-for-sale | Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	617.6	431.3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	21.3	24.7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(2.0)	(1.6)
Available-for-sale Securities, Debt Securities	636.9	454.4
Available For Sale Securities Carrying Value	636.9	454.4
Fixed-maturity securities, available-for-sale | Corporates
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	9,345.5	9,314.7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	499.2	288.7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(48.9)	(185.1)
Available-for-sale Securities, Debt Securities	9,795.8	9,418.3
Available For Sale Securities Carrying Value	9,795.8	9,418.3
Fixed-maturity securities, available-for-sale | Hybrids
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,279.1	412.6
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	52.2	19.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(15.2)	(3.3)
Available-for-sale Securities, Debt Securities	1,316.1	428.8
Available For Sale Securities Carrying Value	1,316.1	428.8
Fixed-maturity securities, available-for-sale | Municipals
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,149.9	998.8
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	116.2	49.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(6.3)	(40.8)
Available-for-sale Securities, Debt Securities	1,259.8	1,007.0
Available For Sale Securities Carrying Value	1,259.8	1,007.0
Fixed-maturity securities, available-for-sale | Agency Residential Mortgage Backed Securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	104.3	96.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3.1	2.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(0.1)	(0.3)
Available-for-sale Securities, Debt Securities	107.3	98.6
Available For Sale Securities Carrying Value	107.3	98.6
Fixed-maturity securities, available-for-sale | Non-agency residential mortgage-backed securities
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,880.5	1,304.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	137.2	77.4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(11.0)	(13.4)
Available-for-sale Securities, Debt Securities	2,006.7	1,368.0
Available For Sale Securities Carrying Value	2,006.7	1,368.0
Fixed-maturity securities, available-for-sale | U.S. Government
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	291.0	998.5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	6.4	7.2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1.4)	(3.9)
Available-for-sale Securities, Debt Securities	296.0	1,001.8
Available For Sale Securities Carrying Value	296.0	1,001.8
Asset based loan receivables [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	811.6	560.4
Other Investments Gross Unrealized Gain
Other Short Term Investments Gross Unrealized Losses
Trading Securities	811.6	560.4
Derivative Financial Instruments, Assets [Member]
Schedule of Investments [Line Items]
Trading Securities, Cost	177.7	141.7
Other Investments Gross Unrealized Gain	123.3	88.5
Other Short Term Investments Gross Unrealized Losses	(4.7)	(8.4)
Trading Securities	296.3	221.8
Other invested assets
Schedule of Investments [Line Items]
Trading Securities, Cost		31.2
Other Investments Gross Unrealized Gain
Other Short Term Investments Gross Unrealized Losses
Trading Securities	165.0	31.2
Other invested assets | Other invested assets | Policy loans and other invested assets
Schedule of Investments [Line Items]
Trading Securities, Cost	164.9
Other Investments Gross Unrealized Gain	$ 0.1