Risks and Uncertainties Risks and Uncertainties - Additional Information (Details) - USD ($) $ in Millions	Mar. 31, 2016	Sep. 30, 2015
Concentration Risk [Line Items]
Industry Holdings, Number of Top Investments	10
Asset Backed Loans And Other Invested Assets Gross	$ 184.6	$ 274.6
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments	10.00%
Allowance for Doubtful Accounts Receivable, Current	$ 47.6	$ 44.0
Credit Concentration Risk [Member]
Concentration Risk [Line Items]
Percentage Of Accounts Receivable Major Customers	13.00015%	12.67183%
Fair Value, Concentration of Risk, All Financial Instruments [Member]
Concentration Risk [Line Items]
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio	32.60%	16.50%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Financial Services Sector [Member]
Concentration Risk [Line Items]
Number Of Investment Holdings In Industry	105	107
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments	0.00%	0.00%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Energy, Mining and Metals Sector [Member]
Concentration Risk [Line Items]
Number Of Investment Holdings In Industry	93	98
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments	0.00%	0.00%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Funds Withheld Receivables [Member] | Financial Services Sector [Member]
Concentration Risk [Line Items]
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio	15.40%	15.80%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Funds Withheld Receivables [Member] | Energy, Mining and Metals Sector [Member]
Concentration Risk [Line Items]
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio	11.60%	13.80%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Funds Withheld Receivables [Member] | Asset-backed Securities [Member] | Financial Services Sector [Member]
Concentration Risk [Line Items]
Investment Securities Held by Subsidiaries Subject to Specialized Industry Accounting Principles at Fair Value	$ 258.1	$ 269.7
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Funds Withheld Receivables [Member] | Asset-backed Securities [Member] | Energy, Mining and Metals Sector [Member]
Concentration Risk [Line Items]
Investment Securities Held by Subsidiaries Subject to Specialized Industry Accounting Principles at Fair Value	$ 194.9	$ 236.6