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Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Other-than-temporary impairments $ (0.7) $ 0.0 $ 0.0
Catastrophe bonds — net unrealized (losses)/gains (2.4) 0.0 (0.3)
Net change in gross unrealized gains (losses) 60.3 22.5 (33.1)
Total net realized and unrealized investment gains recorded in the statement of operations 120.5 45.2 17.0
Change in available for sale net unrealized gains:      
Fixed income securities (14.8) (39.0) (82.2)
Short-term investments 0.0 0.0 (0.1)
Equity securities 0.0 0.0 (27.3)
Total change in pre-tax available for sale unrealized gains (14.8) (39.0) (109.6)
Change in taxes 2.0 1.3 4.4
Total change in net unrealized gains, net of taxes recorded in other comprehensive income (12.8) (37.7) (105.2)
Fixed income securities — Available for sale      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Available for sale securities — gross realized gains 10.2 18.6 11.7
Available for sale securities — gross realized (losses) (6.6) (8.3) (2.7)
Cash and cash equivalents — gross realized gains 0.4 0.2 0.0
Cash and cash equivalents — gross realized (losses) (0.1) (0.6) 0.0
Equity securities — Available for sale      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Available for sale securities — gross realized gains 0.0 0.0 31.9
Available for sale securities — gross realized (losses) 0.0 0.0 (3.0)
Trading securities — gross realized gains 59.0 54.1 46.0
Trading securities — gross realized (losses) (13.7) (46.3) (31.7)
Fixed income securities — Trading      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Cash and cash equivalents — gross realized gains 1.3 0.1 0.0
Cash and cash equivalents — gross realized (losses) 0.0 (0.3) 0.0
Trading securities — gross realized gains 9.7 12.6 4.9
Trading securities — gross realized (losses) (4.5) (7.3) (6.1)
Short-term Investments      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Available for sale securities — gross realized gains 0.1 0.0 0.0
Trading securities — gross realized gains 2.7 0.0 0.0
MVI      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Loss from equity method investment 0.1 0.3 0.0
Chaspark      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Loss from equity method investment (0.9) (0.3) 0.6
Bene      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Loss from equity method investment 0.3 0.1 0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | AgriLogic      
Schedule of Investment Income, Reported Amounts, by Category [Line Items]      
Gross realized gain on sale of AgriLogic $ 4.3 $ 0.0 $ 0.0