Fair Value Measurements - Quantitative Information for Level Three Inputs (Details) - Market Yield [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Collateralized Loan Obligations
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 128.1	$ 93.2
Collateralized Loan Obligations | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	3.40%	4.30%
Collateralized Loan Obligations | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	10.10%	10.60%
Collateralized Loan Obligations | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	7.00%	7.80%
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 14.8	$ 13.2
Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	519.4	494.8
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 104.0	$ 96.2
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	2.80%	3.00%
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	8.90%	6.70%
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	4.40%	3.80%
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 128.0	$ 138.1
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	4.70%	4.50%
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	19.40%	15.70%
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	9.80%	10.00%
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 144.5	$ 154.1
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	9.90%	9.60%
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	24.80%	24.30%
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	13.00%	12.90%