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	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Other-than-temporary impairments	$ 0.0	$ (0.1)	$ 0.0	$ (0.4)
Catastrophe bonds	0.0	(0.1)	0.9	(0.1)
Net change in gross unrealized (losses) gains	(14.9)	38.5	(123.7)	79.4
Total net realized and unrealized investment (losses) gains recorded in the statement of operations	(20.7)	42.0	(58.4)	88.2
Change in available for sale net unrealized (losses) gains:
Fixed income securities	(22.8)	12.9	(105.7)	14.9
Total change in pre-tax available for sale unrealized (losses) gains	(22.8)	12.9	(105.7)	14.9
Change in taxes	1.2	(1.1)	6.7	(1.0)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(21.6)	11.8	(99.0)	13.9
Total fixed income securities — Available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	2.6	3.3	4.4	5.5
Fixed income maturities — gross realized (losses)	(5.6)	(2.4)	(7.4)	(3.7)
Cash and cash equivalents - gross realized gains	0.0	0.0	0.2	0.0
Cash and cash equivalents — gross realized gains (losses)	(0.3)	(0.1)	(0.4)	(0.1)
Fixed Income Maturities - Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Cash and cash equivalents — gross realized gains (losses)	(0.1)	0.0	1.5	0.0
Fixed income securities - gross realized gains	1.0	3.5	2.6	5.3
Fixed income securities - gross realized (losses)	(3.4)	(0.4)	(10.4)	(2.4)
Equity securities — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Trading securities, gross realized gain	0.0	4.4	94.5	8.9
Equity securities — gross realized (losses)	0.2	(3.6)	(20.1)	(5.0)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income maturities — gross realized gains	0.0	0.0	0.0	0.1
Trading securities, gross realized gain	0.0	0.1	0.0	0.1
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	(0.1)	(0.1)	(0.2)	(0.1)
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	0.0	(0.8)	0.0	0.9
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Change in fair value of investment	$ (0.1)	$ (0.2)	$ (0.3)	$ (0.2)

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

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Cash and Cash Equivalents Gross Realized Gains

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Trading Securities, Catastrophe Bonds Gain (Loss)

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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

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