Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, available for sale — gross realized gains	$ 0.8	$ 2.3	$ 5.2	$ 7.8
Fixed income securities, available for sale — gross realized (losses)	(1.1)	(1.6)	(8.5)	(5.3)
Other-than-temporary impairments	0.0	(0.1)	0.0	(0.5)
Fixed income securities, trading - gross realized gains	1.0	3.0	3.6	8.3
Fixed income securities, trading - gross realized (losses)	(4.1)	(0.2)	(14.5)	(2.6)
Equity securities — gross realized gains	0.0	46.1	94.5	55.0
Equity securities — gross realized (losses)	0.0	(7.4)	(20.1)	(12.4)
Catastrophe bonds	0.0	(3.1)	0.9	(3.2)
Net change in gross unrealized gains (losses)	4.5	(25.0)	(119.2)	54.4
Total net realized and unrealized investment (losses) gains recorded in the statement of operations	(0.9)	17.5	(59.3)	105.7
Change in available for sale net unrealized (losses) gains:
Fixed income securities	(21.0)	0.3	(126.7)	15.2
Total change in pre-tax available for sale unrealized (losses) gains	(21.0)	0.3	(126.7)	15.2
Change in taxes	1.3	(0.4)	8.0	(1.4)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(19.7)	(0.1)	(118.7)	13.8
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, available for sale — gross realized gains	0.0	0.0	0.0	0.1
Fixed income securities, available for sale — gross realized (losses)	(0.1)	0.0	(0.1)	0.0
Fixed income securities, trading - gross realized gains	0.0	1.9	0.0	2.0
Fixed income securities, trading - gross realized (losses)	(1.4)	0.0	(1.4)	0.0
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, available for sale — gross realized gains	0.0	0.3	0.2	0.3
Fixed income securities, available for sale — gross realized (losses)	0.0	0.0	(0.4)	(0.1)
Fixed income securities, trading - gross realized gains	0.0	1.3	1.5	1.3
Fixed income securities, trading - gross realized (losses)	(0.2)	0.0	(0.2)	0.0
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0.0	0.0	(0.2)	(0.1)
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0.0	0.0	0.0	0.9
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	$ (0.3)	$ 0.0	$ (0.6)	$ (0.2)

Total change in pre-tax available for sale unrealized gains (losses)

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Trading Securities, Catastrophe Bonds Gain (Loss)

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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