Fair Value of Financial Instruments - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Millions		Sep. 30, 2016	Sep. 30, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 1,650.4
Asset Backed Loans And Other Invested Assets		35.0	$ 226.7
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure And Measurement	[1]	0.0	0.0
Financial Liabilities Fair Value Disclosure	[1]	0.0	0.0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Fair Value Disclosure And Measurement	[1]	0.0	0.0
Debt Instrument, Fair Value Disclosure	[1],[2]	5,700.1	6,071.0
Financial Liabilities Fair Value Disclosure	[1]	5,700.1	6,071.0
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	[1]	8.5
Asset Backed Loans And Other Invested Assets	[1]	35.0	226.7
Assets Fair Value Disclosure And Measurement	[1]	43.5	238.2
Debt Instrument, Fair Value Disclosure	[1],[2]	29.1	99.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	922.9	960.3
Financial Liabilities Fair Value Disclosure	[1]	952.0	1,059.4
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	[1]	8.5
Asset Backed Loans And Other Invested Assets	[1]	35.0	226.7
Assets Fair Value Disclosure And Measurement	[1]	43.5	238.2
Debt Instrument, Fair Value Disclosure	[1],[2]	5,729.2	6,170.1
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	922.9	960.3
Financial Liabilities Fair Value Disclosure	[1]	6,652.1	7,130.4
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]	8.5
Asset Backed Loans And Other Invested Assets	[1]	35.0	226.7
Assets Fair Value Disclosure And Measurement	[1]	43.5	238.2
Debt Instrument, Fair Value Disclosure	[1],[2]	5,430.9	5,984.6
Liabilities Related to Investment Contracts, Fair Value Disclosure	[1]	988.3	1,084.5
Financial Liabilities Fair Value Disclosure	[1]	6,419.2	7,069.1
Other invested assets | 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	[1]		2.5
Other invested assets | 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	[1]		2.5
Other invested assets | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]		2.5
Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		8.5
Policy Loans [Member] | 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	[1]		9.0
Policy Loans [Member] | 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	[1]		9.0
Policy Loans [Member] | Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Policy loans and other invested assets	[1]		$ 9.0
Fixed Maturities Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		$ 1,526.9

[1] The carrying amounts of cash and cash equivalents, trade receivables, accounts payable and accrued investment income approximate fair value due to their short duration and, accordingly, they are not presented in the tables above. (b) The fair values of debt set forth above are generally based on quoted or observed market prices.

[2] The fair values of debt set forth above are generally based on quoted or observed market prices.