Fair Value of Financial Instruments - Financial Assets and Liabilities Not Measured at Fair Value (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Document Period End Date	Sep. 30, 2014
Asset Backed Loans And Other Invested Assets	$ 811.6	$ 560.4
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Asset Backed Loans And Other Invested Assets
Assets Fair Value Disclosure And Measurement	1,319.2	1,899.7
Debt Instrument, Fair Value Disclosure
Redeemable preferred stock, excluding equity conversion feature
Liabilities Related to Investment Contracts, Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Asset Backed Loans And Other Invested Assets
Assets Fair Value Disclosure And Measurement	0	0
Debt Instrument, Fair Value Disclosure	5,308.5	4,773.2
Redeemable preferred stock, excluding equity conversion feature
Liabilities Related to Investment Contracts, Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	5,308.5	4,773.2
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Asset Backed Loans And Other Invested Assets	811.6	560.4
Assets Fair Value Disclosure And Measurement	963.3	591.6
Debt Instrument, Fair Value Disclosure
Redeemable preferred stock, excluding equity conversion feature		377.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,108.8	12,378.6
Financial Liabilities Fair Value Disclosure	13,108.8	12,755.7
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Asset Backed Loans And Other Invested Assets	811.6	560.4
Assets Fair Value Disclosure And Measurement	2,282.5	2,491.3
Debt Instrument, Fair Value Disclosure	5,308.5	4,773.2
Redeemable preferred stock, excluding equity conversion feature		377.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	13,108.8	12,378.6
Financial Liabilities Fair Value Disclosure	18,417.3	17,528.9
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,319.2	1,899.7
Asset Backed Loans And Other Invested Assets	811.6	560.4
Assets Fair Value Disclosure And Measurement	2,282.5	2,491.3
Debt Instrument, Fair Value Disclosure	5,157.8	4,896.1
Redeemable preferred stock, excluding equity conversion feature		329.4
Liabilities Related to Investment Contracts, Fair Value Disclosure	14,555.4	13,703.8
Financial Liabilities Fair Value Disclosure	19,713.2	18,929.3
Policy loans | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets
Policy loans | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets
Policy loans | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	151.7	31.2
Policy loans | Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	151.7	31.2
Policy loans | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	$ 151.7	$ 31.2