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Investments - Other Investments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	$ 66.4
Unrealized/realized gain/(loss)	(0.3)
Closing value of investment	66.1
MVI
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.5
Unrealized/realized gain/(loss)	(0.1)
Closing value of investment	0.4
Bene
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	2.9
Unrealized/realized gain/(loss)	(0.2)
Closing value of investment	2.7
Digital Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	0.5
Unrealized/realized gain/(loss)	0.0
Closing value of investment	0.5
Crop Re
Equity Method Investment, Aggregate Cost [Roll Forward]
Opening undistributed value of investment	62.5
Unrealized/realized gain/(loss)	0.0
Closing value of investment	$ 62.5

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- Definition

Equity Method Investment, Aggregate Cost [Roll Forward]

+ References

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- Definition

This element represents the aggregate cost of investments accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

This item represents the amount of unrealized intercompany profits and losses on transactions between the Entity and unconsolidated subsidiaries (including certain corporate joint ventures, and noncontrolled corporations), accounted for under the equity method of accounting, which have not been eliminated in preparing the Entity's financial statements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.3A-04)
-URI http://asc.fasb.org/extlink&oid=27015204&loc=d3e355119-122828

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 35
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99375318&loc=d3e32367-111569

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