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, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	$ 68.8	$ 14.4	$ 6.4
Fixed income securities - gross realizes (losses)	(1.8)	(7.3)	(11.4)
Net change in expected credit gains (losses)	0.4	0.0	0.0
Other-than-temporary impairments		0.0
Fixed income securities, trading - gross realized gains	18.2	34.3	4.6
Fixed income securities, trading, gross realizes (losses)	(3.8)	(2.6)	(25.0)
Equity securities - gross realized gains	0.0	0.0	94.5
Equity securities - gross realized (losses)	0.0	0.0	(20.1)
Catastrophe bonds	0.0	0.9	2.2
Net change in gross unrealized gains / (losses)	0.0	47.2	(112.1)
Total net realized and unrealized investment gains/(losses) recorded in the statement of operations	71.1	86.2	(64.7)
Change in available for sale net unrealized gain/(losses):
Fixed income securities	108.5	164.9	(81.3)
Income tax (expense)/benefit	(6.2)	(13.6)	4.8
Total change in net unrealized gains/(losses), net of taxes recorded in other comprehensive income	102.3	151.3	(76.5)
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, trading - gross realized gains	0.0	0.0	0.1
Fixed income securities, trading, gross realizes (losses)	0.0	0.0	(4.2)
Cash and cash equivalents
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities - gross realized gains	0.5	0.1	0.3
Fixed income securities - gross realizes (losses)	(0.4)	(0.2)	(0.5)
Fixed income securities, trading - gross realized gains	0.2	0.0	1.5
Fixed income securities, trading, gross realizes (losses)	(0.3)	(0.3)	(0.3)
Privately-held investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Fixed income securities, trading - gross realized gains	0.0	0.2	0.0
Fixed income securities, trading, gross realizes (losses)	0.0	(0.2)	0.0
Net change in gross unrealized gains / (losses)	20.4	0.0	0.0
MVI
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	(0.4)	(0.1)	(0.2)
Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	0.0	0.0	(0.9)
Digital Re
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Income/(loss) from equity method investment	(0.3)	(0.2)	0.4
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Crop Re
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross realized gain on sale of equity method investment	8.6	0.0	0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Bene
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Gross realized gain on sale of equity method investment	$ 1.8	$ 0.0	$ 0.0

Available for sale securities change in unrealized gains.

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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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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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