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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	$ 6.4	$ 10.2	$ 18.6
Fixed income securities - gross realizes (losses)	(11.4)	(6.6)	(8.3)
Other-than-temporary impairments	0.0	(0.7)	0.0
Fixed income securities, trading - gross realized gains	4.6	9.7	12.6
Fixed income securities, trading, gross realizes (losses)	(25.0)	(4.5)	(7.3)
Equity securities - gross realized gains	94.5	59.0	54.1
Equity securities - gross realized (losses)	(20.1)	(13.7)	(46.3)
Catastrophe bonds	2.2	(2.4)	0.0
Net change in gross unrealized (losses) gains	(112.1)	60.3	22.5
Total net realized and unrealized investment (losses)/gains recorded in the statement of operations	(64.7)	120.5	45.2
Change in available for sale net unrealized (losses):
Fixed income securities	(81.3)	(14.8)	(39.0)
Change in taxes	4.8	2.0	1.3
Total change in net unrealized (losses), net of taxes recorded in other comprehensive income	(76.5)	(12.8)	(37.7)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	0.0	0.1	0.0
Fixed income securities - gross realizes (losses)	0.0	0.0	0.0
Fixed income securities, trading - gross realized gains	0.1	2.7	0.0
Fixed income securities, trading, gross realizes (losses)	(4.2)	0.0	0.0
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	0.3	0.4	0.2
Fixed income securities - gross realizes (losses)	(0.5)	(0.1)	(0.6)
Fixed income securities, trading - gross realized gains	1.5	1.3	0.1
Fixed income securities, trading, gross realizes (losses)	(0.3)	0.0	(0.3)
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	(0.2)	(0.1)	(0.3)
Chaspark
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	0.0	0.9	0.3
Digital Re
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	0.4	0.0	0.0
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	(0.9)	(0.3)	(0.1)
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	$ 0.0	$ 4.3	$ 0.0

Available for sale securities change in unrealized gains.

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Net Realized And Unrealized Investment 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 of realized gain from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of realized loss from sale of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of gain (loss) from sale and disposal of integrated set of activities and assets capable of being conducted and managed for purpose of providing return in form of dividend, lower cost, or other economic benefit to investor, owner, member and participant.

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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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