Fair Value Measurements - Quantitative Information for Level Three Inputs (Details) - Market Yield [Member] - Fair Value, Inputs, Level 3 [Member] $ in Millions	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 466.7	$ 93.2
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	14.5	13.2
Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	860.0	494.8
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	136.3	96.2
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	151.6	138.1
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 90.9	$ 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	0.030	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	0.105	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	0.050	0.038
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.035	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	0.051	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	0.095	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	0.278	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	0.130	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	0.037	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	0.233	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	0.097	0.100