Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Market Approach Valuation Technique [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Collateralized Loan Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 87.3	$ 64.4
Collateralized Loan Obligations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	3.10%	2.70%
Collateralized Loan Obligations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	10.80%	8.10%
Collateralized Loan Obligations [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	6.10%	5.20%
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 14.7	$ 21.6
Investments in Fixed Maturities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	531.2	435.6
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 98.7	$ 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	4.40%	3.90%
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 114.2	$ 92.7
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	5.90%	3.80%
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	15.30%	14.50%
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	10.40%	10.60%
External Credit Rating, Non Investment Grade [Member] | Junior Subordinated Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 216.3	$ 149.9
External Credit Rating, Non Investment Grade [Member] | Junior Subordinated 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 Subordinated Debt [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	26.20%	21.00%
External Credit Rating, Non Investment Grade [Member] | Junior Subordinated Debt [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	13.60%	13.40%