Investment Securities (Other long-term investments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 130.6	$ 125.3
Equity method eligible private equity securities, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	108.4	83.2
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	0.0	(3.7)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	14.1	4.0
White Mountains
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	253.1	208.8
White Mountains \| Equity method eligible private equity securities, at fair value
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 84.7	$ 58.0
OneTitle
Schedule of Trading Securities and Other Trading Assets [Line Items]
Ownership interest (as a percent)	20.00%
Surplus notes	$ 10.0

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66011266&loc=d3e33918-111571

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Amount of investments classified as other.

+ 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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f,g))
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Financial instruments issued by insurance entities (surplus notes) that are includable in surplus for statutory accounting purposes as prescribed or permitted by state laws and regulations that are liabilities of the issuer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6486086&loc=d3e24387-158525

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