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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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, available for sale — gross realized gains	$ 3.0	$ 1.8
Fixed income securities, available for sale — gross realized (losses)	(3.2)	(1.8)
Other-than-temporary impairments	0.0	0.0
Fixed income securities, trading - gross realized gains	2.3	1.6
Fixed income securities, trading - gross realized (losses)	(2.8)	(7.0)
Equity securities — gross realized gains	0.0	94.5
Equity securities — gross realized (losses)	0.0	(20.3)
Catastrophe bonds	0.3	0.9
Net change in gross unrealized gains/(losses)	36.3	(108.8)
Total net realized and unrealized investment gains/(losses) recorded in the statement of operations	35.8	(37.7)
Change in available for sale net unrealized gains/(losses):
Fixed income securities	75.6	(82.9)
Change in taxes	(5.2)	5.5
Total change in net unrealized gains/(losses), net of taxes, recorded in other comprehensive income	70.4	(77.4)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, trading - gross realized (losses)	(0.1)	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	0.2
Fixed income securities, available for sale — gross realized (losses)	0.0	(0.1)
Fixed income securities, trading - gross realized gains	0.1	1.6
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0.0	(0.1)
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	$ (0.1)	$ (0.2)

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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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