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Summary of Significant Accounting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) publisher advertiser	Sep. 30, 2016 USD ($)	Sep. 28, 2017 $ / shares	Jun. 30, 2017	Dec. 31, 2016	Jul. 31, 2012 USD ($)
HG Global
Basis of Presentation
Percentage of par value of policy reinsured	15.00%
Surplus notes						$ 503.0
MediaAlpha
Basis of Presentation
Number of advertisers | advertiser	330
Number of publishers | publisher	280
Preferred Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	96.90%			96.90%	96.90%
Common Stock | HG Global
Basis of Presentation
Ownership interest (as a percent)	88.40%			88.40%	88.40%
Restricted Stock
Basis of Presentation
Payments related to tax withholding for share-based compensation	$ 9.3	$ 5.8
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
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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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