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Fair Value Measurements - Quantitative Information for Level Three Inputs (Details) - Market Yield [Member] - Fair Value, Inputs, Level 3 [Member]
$ in Millions
Jun. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Collateralized Loan Obligations    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure $ 95.9 $ 93.2
Other Debt Obligations [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure 16.4 13.2
Debt Securities [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure 481.4 494.8
External Credit Rating, Investment Grade [Member] | Private Placement [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure 117.4 96.2
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure 148.3 138.1
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Assets, Fair Value Disclosure $ 103.4 $ 154.1
Measurement Input, Discount Rate [Member] | External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Alternative Investment, Measurement Input 0.029 0.030
Measurement Input, Discount Rate [Member] | External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Alternative Investment, Measurement Input 0.089 0.067
Measurement Input, Discount Rate [Member] | External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Weighted Average [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Alternative Investment, Measurement Input 0.046 0.038
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.040 0.043
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.104 0.106
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Weighted Average [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.081 0.078
Junior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.103 0.096
Junior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.248 0.243
Junior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Weighted Average [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.127 0.129
Senior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Minimum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.048 0.045
Senior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Maximum [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.195 0.157
Senior Debt [Member] | Measurement Input, Discount Rate [Member] | External Credit Rating, Non Investment Grade [Member] | Weighted Average [Member]    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Debt Instrument, Measurement Input 0.099 0.100