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	6 Months Ended
	Mar. 31, 2014		Sep. 30, 2013		Dec. 31, 2012	Sep. 30, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Document Period End Date	Mar. 31, 2014
Cash and cash equivalents	$ 1,319.8		$ 1,899.7		$ 1,475.0	$ 1,470.7
Derivatives	273.0		221.8
Equity conversion feature of preferred stock	364.8		330.8
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		[1],[2]		[1],[2]
Contingent purchase price reduction receivable	41.5	[1]	41.0	[1]
Funds Withheld Receivable	0	[1]
Assets, Fair Value Disclosure, Recurring	755.6	[1]	523.5
Equity conversion feature of preferred stock	364.8	[1]	330.8	[1]
Liabilities, Fair Value Disclosure, Recurring	2,234.5	[1]	1,875.2	[1]
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,319.8	[1],[2]	1,899.7	[1],[2]
Contingent purchase price reduction receivable				[1]
Funds Withheld Receivable	19.0	[1]
Assets, Fair Value Disclosure, Recurring	1,641.3	[1]	2,761.4
Equity conversion feature of preferred stock		[1]		[1]
Liabilities, Fair Value Disclosure, Recurring	0	[1]	0	[1]
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		[1],[2]	0	[1],[2]
Contingent purchase price reduction receivable				[1]
Funds Withheld Receivable	135.5	[1]
Assets, Fair Value Disclosure, Recurring	16,509.2	[1]	14,536.0
Equity conversion feature of preferred stock		[1]		[1]
Liabilities, Fair Value Disclosure, Recurring	12.3	[1]	13.4	[1]
Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	1,319.8	[1],[2]	1,899.7	[1],[2]
Contingent purchase price reduction receivable	41.5	[1]	41.0	[1]
Funds Withheld Receivable	154.5	[1]
Assets, Fair Value Disclosure, Recurring	18,906.1	[1]	17,820.9
Equity conversion feature of preferred stock	364.8	[1]	330.8	[1]
Liabilities, Fair Value Disclosure, Recurring	2,246.8	[1]	1,888.6	[1]
Commodity Option [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		[1]		[1]
Derivative Liability		[1]		[1]
Commodity Option [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		[1]		[1]
Derivative Liability		[1]		[1]
Commodity Option [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0.4	[1]	4.1	[1]
Derivative Liability	5.1	[1]	2.4	[1]
Commodity Option [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	0.4	[1]	4.1	[1]
Derivative Liability	5.1	[1]	2.4	[1]
Fixed Index Annuity [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1,718.7	[1]	1,544.4	[1]
Fixed Index Annuity [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		[1]		[1]
Fixed Index Annuity [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		[1]		[1]
Fixed Index Annuity [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	1,718.7	[1]	1,544.4	[1]
Front Street Future Policyholder Benefits Reserve [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	151.0	[1]
Front Street Future Policyholder Benefits Reserve [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		[1]
Front Street Future Policyholder Benefits Reserve [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability		[1]
Front Street Future Policyholder Benefits Reserve [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	151.0	[1]
FX embedded derivative included in asset-based loans [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		[1]
FX embedded derivative included in asset-based loans [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives		[1]
FX embedded derivative included in asset-based loans [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	1.2	[1]
FX embedded derivative included in asset-based loans [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivatives	1.2	[1]
Equity [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	10.8	[1]	10.7	[2]
Equity [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	121.0	[1]	70.8	[2]
Equity [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading		[1]		[2]
Equity [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	131.8	[1]	81.5	[2]
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading		[1]
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	0	[1]
Other Invested Assets [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	2.1	[1]
Other Invested Assets [Member] | Fair Value [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities - trading	$ 2.1	[1]

[1] The carrying amounts of trade receivables, accounts payable, accrued investment income and portions of other insurance liabilities approximate fair value due to their short duration and, accordingly, they are not presented in the tables above.

[2] The fair values of cash equivalents and equity investments set forth above are generally based on quoted or observed market prices.