Fair Value of Financial Instruments - Financial Assets and Liabilities Not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Instrument, Fair Value Disclosure	$ 5,908.0	$ 5,700.1
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Backed Loans And Other Invested Assets		33.3
Debt Instrument, Fair Value Disclosure	92.0	121.9
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Backed Loans And Other Invested Assets		33.3
Debt Instrument, Fair Value Disclosure	6,000.0	5,822.0
Reported Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Backed Loans And Other Invested Assets		33.3
Debt Instrument, Fair Value Disclosure	$ 5,774.1	$ 5,525.8