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Investments - Additional Information (Narrative) (Details) $ in Millions			1 Months Ended	12 Months Ended
	Dec. 18, 2017 USD ($)	Jul. 26, 2016 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) municipal_security	Dec. 31, 2015 USD ($)	Jan. 01, 2017 USD ($)	Oct. 02, 2012
Gain (Loss) on Investments [Line Items]
Held capital requirement, percentage				20.00%	100.00%
Catastrophe bonds, trading, cost				$ 32.4	$ 42.5
Investment in the period				64.9	3.3
Proceeds from sale of equity method investments				(9.3)	0.0	$ 0.0
Equity method investment, aggregate cost				66.4	$ 12.1	8.9
Investment funded				0.0
Number of municipal holdings | municipal_security					1
Municipal holdings, fair value					$ 0.1
Other-than-temporary impairment charge				$ 0.7	$ 0.0	0.0
Equity securities
Gain (Loss) on Investments [Line Items]
Percentage of investment				5.80%
BBB Emerging Market Debt
Gain (Loss) on Investments [Line Items]
Percentage of investment				4.00%
Risk Asset Portfolio Cash
Gain (Loss) on Investments [Line Items]
Percentage of investment				0.20%
BBB Emerging Market Debt And Risk Portfolio Cash
Gain (Loss) on Investments [Line Items]
Percentage of investment				10.00%	10.80%
Bene
Gain (Loss) on Investments [Line Items]
Investment in the period		$ 3.3		$ 0.0	$ 3.3
Gross realized and unrealized gains (loss)				(0.3)	(0.1)	0.0
Proceeds from sale of equity method investments				0.0
Equity method investment, ownership percentage		20.00%
Equity method investment, aggregate cost				2.9	3.2	0.0
Chaspark
Gain (Loss) on Investments [Line Items]
Investment in the period				0.0	0.0
Percentage of ownership acquired								58.50%
Gross realized and unrealized gains (loss)				0.9	0.3	(0.6)
Proceeds from sale of equity method investments				(9.3)
Equity method investment, aggregate cost				0.0	8.4	8.1
MVI
Gain (Loss) on Investments [Line Items]
Investment in the period			$ 0.8	0.1	0.0
Gross realized and unrealized gains (loss)				(0.1)	(0.3)	0.0
Proceeds from sale of equity method investments				0.0
Equity method investment, aggregate cost				0.5	0.5	0.8
Digital Re
Gain (Loss) on Investments [Line Items]
Investment in the period				2.3	0.0
Proceeds from sale of equity method investments				0.0
Equity method investment, ownership percentage							49.00%
Equity method investment, aggregate cost				0.5	0.0	0.0	$ 2.3
Crop Re
Gain (Loss) on Investments [Line Items]
Investment in the period				62.5	0.0
Proceeds from sale of equity method investments				0.0
Equity method investment, ownership percentage	23.20%
Equity method investment, aggregate cost	$ 62.5			$ 62.5	0.0	$ 0.0
Equity method investment, cash payment	$ 5.9
Aspen Bermuda Limited [Member]
Gain (Loss) on Investments [Line Items]
Transfer from Investments					$ 95.1
CGB Insurance Diversified Services, Inc. | Crop Re
Gain (Loss) on Investments [Line Items]
Equity method investment, counterparty ownership percentage	76.80%

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