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Funds Withheld Receivables Additional information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Mar. 31, 2017
Mar. 31, 2016
Sep. 30, 2016
Document Period End Date     Mar. 31, 2017    
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year 135   135   146
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities $ 0.0 $ 4.5 $ 1.0 $ 5.9  
Insurance Segment [Member]          
Available For Sale Depressed Securities Fair Value Disclosure 21.7   21.7   $ 39.6
Asset Backed Securities and Corporate Securities [Member]          
Available-for-sale Securities, Amortized Cost Basis 12.5 8.7 12.5 8.7  
Available-for-sale Securities $ 11.5 $ 2.8 $ 11.5 $ 2.8  
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Energy, Mining and Metals Sector [Member]          
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments 42.00%   42.00%   43.40%
Fair Value, Concentration of Risk, All Financial Instruments [Member] | Financial Services Sector [Member]          
Equity Securities Holdings Exposure Amount of Single Issuer Greater than Stated Percentage of Total Investments 47.70%   47.70%   48.00%
Funds Withheld Receivables [Member] | Fair Value, Concentration of Risk, All Financial Instruments [Member] | Energy, Mining and Metals Sector [Member]          
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio 10.30%   10.30%   11.40%
Funds Withheld Receivables [Member] | Fair Value, Concentration of Risk, All Financial Instruments [Member] | Financial Services Sector [Member]          
Percentage Of Investments Fair Value In Relation To Invested Assets Portfolio 14.90%   14.90%   14.10%