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Risks and Uncertainties Risks and Uncertainties - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Sep. 30, 2016
Concentration Risk [Line Items]      
Document Period End Date Dec. 31, 2016    
Industry Holdings, Number of Top Investments 10    
Allowance for Doubtful Accounts Receivable, Current $ (48.0)   $ (46.8)
Credit Concentration Risk [Member]      
Concentration Risk [Line Items]      
Percentage Of Net Sales Due To Customers 11.60% 13.10%  
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Financial Services Sector [Member]      
Concentration Risk [Line Items]      
Number Of Investment Holdings In Industry 87   81
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments 46.40%   48.00%
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 73   74
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments 42.80%   43.40%
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%   14.10%
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 11.80%   11.40%
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 $ 247.0   $ 232.8
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 $ 189.7   $ 188.6