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UNAUDITED CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Fixed income securities, available for sale, amortized cost	$ 5,465.2	$ 5,201.2
Fixed income securities, trading at fair value (amortized cost )	1,652.7	1,634.9
Equity securities, trading at fair value, cost	0.0	414.8
Short-term Investments, available for sale, amortized cost	54.7	90.0
Short-term investments, trading at fair value, amortized cost	27.0	73.0
Catastrophe bonds, trading at fair value, cost	35.0	33.5
Cash and cash equivalents, within consolidated VIEs	$ 103.0	$ 166.6
Ordinary shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
Ordinary shares, issued (in shares)	59,652,826	59,474,085
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
5.95% preference shares
Preference shares, issued (in shares)	11,000,000	11,000,000
Preference shares, rate	5.95%	5.95%
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558
5.625% preference shares
Preference shares, issued (in shares)	10,000,000	10,000,000
Preference shares, rate	5.625%	5.625%
Preference shares, par value (in usd per share)	$ 0.0015144558	$ 0.0015144558

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Cash and Cash Equivalents, within VIEs

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Catastrophe Bonds, Fair Value Disclosure

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Paragraph 1
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