Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) (Market Approach Valuation Technique [Member], Fair Value, Inputs, Level 3 [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Collateralized Loan Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	64.4	44.7
Collateralized Loan Obligations [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	2.70%	3.20%
Collateralized Loan Obligations [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	8.10%	6.80%
Collateralized Loan Obligations [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	5.20%	4.30%
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	21.6	20.1
Investments in Fixed Maturities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	435.6	420.7
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	107.0	108.5
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%	1.00%
External Credit Rating, Investment Grade [Member] | Private Placement [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Inputs, Discount Rate	6.90%	6.00%
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	3.90%	4.30%
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	92.7	93.9
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	3.80%	4.20%
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	14.50%	15.60%
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	10.60%	8.60%
External Credit Rating, Non Investment Grade [Member] | Junior Subordinated Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	149.9	153.5
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%	8.80%
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	21.00%	26.60%
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	13.40%	14.20%