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Basis of Presentation and Significant Accouting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) publisher advertiser	Mar. 31, 2017 USD ($)	Dec. 31, 2017	Sep. 28, 2017 $ / shares	Jul. 31, 2012 USD ($)
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes	$ 499.0				$ 503.0
MediaAlpha
Basis of Presentation
Ownership interest (as a percent)	62.30%		64.40%
Number of advertisers | advertiser	550
Number of publishers | publisher	325
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%		96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%		88.40%
Restricted Stock
Basis of Presentation
Payments related to tax withholding for share-based compensation	$ 8.4	$ 6.5
OneBeacon
Basis of Presentation
Business acquisition, share price | $ / shares				$ 18.10

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 25
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Number of Advertisers

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Percentage of par value of policy reinsured.

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