Fair Value Measurements - Quantitative Information for Level Three Inputs (Details) - Market Yield [Member] - Fair Value, Inputs, Level 3 [Member] $ in Millions	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 528.5	$ 504.9
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	17.2	15.9
Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	950.7	897.4
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	169.6	146.7
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	124.9	142.3
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 110.5	$ 87.6
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	0.028	0.037
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	0.107	0.109
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	0.052
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.043	0.041
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.125	0.134
Collateralized Loan Obligations | Measurement Input, Discount Rate [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.058	0.061
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	0.104	0.110
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	0.169	0.285
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	0.135	0.142
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	0.046	0.048
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	0.164	0.300
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	0.104	0.115