Fair Value Measurements - Quantitative Information about Significant Unobservable Inputs (Details) - Valuation, Market Approach [Member] - Fair Value, Inputs, Level 3 [Member] $ in Millions	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Collateralized Loan Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 504.9	$ 93.2
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	15.9	13.2
Investments in Fixed Maturities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	897.4	494.8
External Credit Rating, Investment Grade [Member] | Private Placement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	146.7	96.2
External Credit Rating, Non Investment Grade [Member] | Senior Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	142.3	138.1
External Credit Rating, Non Investment Grade [Member] | Junior Subordinated Debt [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Assets, Fair Value Disclosure	$ 87.6	$ 154.1
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	0.037	0.030
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	0.109	0.067
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	0.052	0.038
Collateralized Loan Obligations [Member] | Measurement Input, Discount Rate [Member] | Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.041	0.043
Collateralized Loan Obligations [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.134	0.106
Collateralized Loan Obligations [Member] | Measurement Input, Discount Rate [Member] | Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Debt Instrument, Measurement Input	0.061	0.078
Junior Subordinated 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	0.110	0.096
Junior Subordinated 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	0.285	0.243
Junior Subordinated 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	0.142	0.129
Senior Debt Obligations [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	0.048	0.045
Senior Debt Obligations [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	0.300	0.157
Senior Debt Obligations [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	0.115	0.100