Fair Value of Financial Instruments Fair Value of Financial Instruments - Assets and Liabilities Carrying at Fair Value Measured on a Recurring Basis (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Dec. 31, 2014		Sep. 30, 2014		Dec. 31, 2013	Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Document Period End Date	Dec. 31, 2014
Cash and cash equivalents	$ 1,311.4		$ 1,319.2		$ 1,293.8	$ 1,899.7
Derivatives	306.2		296.3
Asset-backed loans	791.9		811.6
Other invested assets	247.0		165.0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-backed loans	791.9		151.7
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,442.9		13,108.8
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total debt	5,842.6	[1]	5,308.5	[1]
Nomura Fund Linked Note [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	11.8
Policy loans [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	233.2		811.6
Other Invested Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities			165.0
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-backed loans	791.9		151.7
Total debt	5,842.6	[1]	5,308.5	[1]
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,442.9		13,108.8
Estimate of Fair Value Measurement [Member] | Policy loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	233.2		811.6
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset-backed loans	791.9		151.7
Total financial assets	2,336.5
Total debt	5,666.0	[1]	5,157.8	[1]
Liabilities Related to Investment Contracts, Fair Value Disclosure	15,019.9		14,555.4
Financial Liabilities Fair Value Disclosure	20,685.9
Reported Value Measurement [Member] | Policy loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	233.2		811.6
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	41.5	[2]	41.5	[2]
Funds Withheld Receivable	512.6	[2]	154.4	[2]
Assets, Fair Value Disclosure, Recurring	19,307.1	[2]	18,501.7	[2]
Liabilities, Fair Value Disclosure, Recurring	2,643.9	[2]	2,062.2	[2]
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable	0	[2]	0	[2]
Assets, Fair Value Disclosure, Recurring	1,160.8	[2]	1,066.0	[2]
Liabilities, Fair Value Disclosure, Recurring	2,636.6	[2]	2,059.4	[2]
Total financial assets	1,025.1		963.3
Financial Liabilities Fair Value Disclosure	13,442.9		13,108.8
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable	0	[2]	0	[2]
Assets, Fair Value Disclosure, Recurring	550.2	[2]	279.3	[2]
Liabilities, Fair Value Disclosure, Recurring	0	[2]	0	[2]
Total financial assets	1,311.4		1,319.2
Financial Liabilities Fair Value Disclosure	0		0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Funds Withheld Receivable	512.6	[2]	154.4	[2]
Assets, Fair Value Disclosure, Recurring	17,596.1	[2]	17,156.4	[2]
Liabilities, Fair Value Disclosure, Recurring	7.3	[2]	2.8	[2]
Total financial assets	0		0
Financial Liabilities Fair Value Disclosure	5,842.6		5,308.5
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	19.9	[2]	3.2	[2]
Derivative Liability	0.9	[2]	0.5	[2]
Fair Value, Measurements, Recurring [Member] | Commodity Option [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	19.9	[2]	3.2	[2]
Derivative Liability	0.9	[2]	0.5	[2]
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	2,140.2	[2]	1,908.1	[2]
Fair Value, Measurements, Recurring [Member] | Fixed Index Annuity [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	2,140.2	[2]	1,908.1	[2]
Fair Value, Measurements, Recurring [Member] | Front Street Future Policyholder Benefits Reserve [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	496.4	[2]	151.3	[2]
Fair Value, Measurements, Recurring [Member] | Future Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1.9
Fair Value, Measurements, Recurring [Member] | Future Contracts [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1.9		0.1
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0.6	[2]	0.6	[2]
Fair Value, Measurements, Recurring [Member] | Interest Rate Contract [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0.6	[2]	0.6	[2]
Fair Value, Measurements, Recurring [Member] | Foreign exchange forward agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives			12.8	[2]
Derivative Liability	2.6	[2]	0.4	[2]
Fair Value, Measurements, Recurring [Member] | Foreign exchange forward agreements [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives			12.8	[2]
Derivative Liability	2.6	[2]	0.4	[2]
Fair Value, Measurements, Recurring [Member] | Call options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	306.2	[2]	296.3	[2]
Fair Value, Measurements, Recurring [Member] | Call options [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	306.2	[2]	296.3	[2]
Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - available-for-sale	625.9	[2]	663.6	[2]
Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - available-for-sale			6.0	[2]
Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - available-for-sale	57.1	[2]	59.2	[2]
Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - available-for-sale	562.7	[2]	598.4	[2]
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1.9	[2]	1.8	[2]
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1.9	[2]	1.8	[2]
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member] | Foreign exchange forward agreements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	17.3
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member] | Foreign exchange forward agreements [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	17.3
Fair Value, Measurements, Recurring [Member] | Equity [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	91.5	[2]	104.5	[2]
Fair Value, Measurements, Recurring [Member] | Equity [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	91.5	[2]	104.5	[2]
Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	13.8		13.3
Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	0		0
Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	2.0	[2]	2.1	[2]
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets	2,336.5		2,282.5
Financial Liabilities Fair Value Disclosure	19,285.5		18,417.3
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Front Street Future Policyholder Benefits Reserve [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	496.4	[2]	151.3	[2]
Fair Value, Measurements, Recurring [Member] | Estimate of Fair Value Measurement [Member] | Future Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability			0.1
Fair Value, Measurements, Recurring [Member] | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total financial assets			2,282.5
Financial Liabilities Fair Value Disclosure			19,713.2
Contingent Purchase Price Reduction Receivable [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent Purchase Price Reduction Receivable Fair Value Disclosure	41.5	[2]	41.5	[2]
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,839.1	[2]	1,792.9	[2]
Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,804.5	[2]	1,755.9	[2]
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	668.4	[2]	636.9	[2]
Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	548.0	[2]	553.8	[2]
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	9,721.4	[2]	9,795.8	[2]
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	907.6	[2]	850.0	[2]
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	8,813.8	[2]	8,945.8	[2]
Hybrids [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,301.5	[2]	1,316.1	[2]
Hybrids [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	[2]	0	[2]
Hybrids [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,301.5	[2]	1,316.1	[2]
Municipals [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,361.9	[2]	1,259.8	[2]
Municipals [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities			37.2	[2]
Municipals [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,323.1	[2]	1,222.6	[2]
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,201.9	[2]	2,114.0	[2]
Residential Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,201.9	[2]	2,114.0	[2]
Broker Quoted [Member] | Asset-backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	34.6	[2]	37.0	[2]
Broker Quoted [Member] | Commercial Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	120.4	[2]	83.1	[2]
Market Pricing [Member] | Equity Securities [Member] | Fair Value, Measurements, Recurring [Member] | Available For Sale Embedded Derivatives [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - available-for-sale	6.1	[2]
Market Pricing [Member] | Municipal Bonds [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	38.8	[2]
Option Pricing [Member] | Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets			11.2
Option Pricing [Member] | Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Other Invested Assets [Member] | Nomura Fund Linked Note [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets	11.8
U.S. Government securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	583.6	[2]	296.0	[2]
U.S. Government securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	401.6	[2]	115.6	[2]
U.S. Government securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 182.0	[2]	$ 180.4	[2]

[1] (a)

[2] December 31, 2014 September 30, 2014 Level 1 Level 2 Level 3 Fair Value Level 1 Level 2 Level 3 Fair ValueAssets Contingent purchase price reduction receivable$— $— $41.5 $41.5 $— $— $41.5 $41.5Derivatives: Interest rate contracts— 0.6 — 0.6 — 0.6 — 0.6Commodity swap and option agreements— 19.9 — 19.9 — 3.2 — 3.2Foreign exchange forward agreements— 17.3 — 17.3 — 12.8 — 12.8Call options and futures contracts— 306.2 — 306.2 — 296.3 — 296.3Fixed maturity securities, available-for-sale: Asset-backed securities— 1,804.5 34.6 1,839.1 — 1,755.9 37.0 1,792.9Commercial mortgage-backed securities— 548.0 120.4 668.4 — 553.8 83.1 636.9Corporates— 8,813.8 907.6 9,721.4 — 8,945.8 850.0 9,795.8Hybrids— 1,301.5 — 1,301.5 — 1,316.1 — 1,316.1Municipals— 1,323.1 38.8 1,361.9 — 1,222.6 37.2 1,259.8Residential mortgage-backed securities— 2,201.9 — 2,201.9 — 2,114.0 — 2,114.0U.S. Government401.6 182.0 — 583.6 115.6 180.4 — 296.0Equity securities: Available-for-sale57.1 562.7 6.1 625.9 59.2 598.4 6.0 663.6Trading91.5 — — 91.5 104.5 — — 104.5Other invested assets— 2.0 11.8 13.8 — 2.1 11.2 13.3Funds withheld receivable (a)— 512.6 — 512.6 — 154.4 — 154.4Total financial assets$550.2 $17,596.1 $1,160.8 $19,307.1 $279.3 $17,156.4 $1,066.0 $18,501.7 Liabilities Derivatives: FIA embedded derivatives, included in contractholder funds$— $— $2,140.2 $2,140.2 $— $— $1,908.1 $1,908.1Front Street future policyholder benefit liability— — 496.4 496.4 — — 151.3 151.3Foreign exchange forward agreements and contracts— 2.6 — 2.6 — 0.1 — 0.1Futures contracts— 1.9 — 1.9 — 0.4 — 0.4Commodity contracts— 0.9 — 0.9 — 0.5 — 0.5Interest rate contracts— 1.9 — 1.9 — 1.8 — 1.8Total financial liabilities$— $7.3 $2,636.6 $2,643.9 $— $2.8 $2,059.4 $2,062.2