Fair Value Measurements - Quantitative Information for Level Three Inputs (Details) (Market Yield [Member], Fair Value, Inputs, Level 3 [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Collateralized Loan Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	69.9	64.4
Collateralized Loan Obligations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	2.50%	2.70%
Collateralized Loan Obligations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	7.70%	8.10%
Collateralized Loan Obligations [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	4.90%	5.20%
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	23.4	21.6
Debt Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	455.0	435.6
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	109.0	107.0
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	2.60%	2.80%
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	6.90%	6.90%
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	3.60%	3.90%
External Credit Rating, Non Investment Grade [Member] | Senior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	93.3	92.7
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	3.60%	3.80%
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	14.50%	14.50%
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	9.90%	10.60%
External Credit Rating, Non Investment Grade [Member] | Junior Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	159.4	149.9
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	8.20%	8.20%
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	21.50%	21.00%
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	13.40%	13.40%