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Summary of Significant Accounting Policies (Basis of Presentation) (Details) $ / shares in Units, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) publisher advertiser | Dec. 31, 2016 USD ($) | Sep. 28, 2017 $ / shares | Dec. 31, 2012 USD ($)
Basis of Presentation |  |  |  |
Net unrealized foreign currency translation losses | $ (1.3) | $ (1.4) |  |
Percentage of par value of policy reinsured | 15.00% |  |  |
Percentage of investments recorded at fair value | 94.00% | 94.00% |  |
HG Global |  |  |  |
Basis of Presentation |  |  |  |
Surplus notes |  |  |  | $ 503.0
MediaAlpha |  |  |  |
Basis of Presentation |  |  |  |
Ownership interest (as a percent) | 64.40% | 60.00% |  |
Number of advertisers | advertiser | 400 |  |  |
Number of publishers | publisher | 300 |  |  |
Preferred stocks | 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% |  |
Intact Financial Corporation | OneBeacon |  |  |  |
Basis of Presentation |  |  |  |
Sale of stock, price per share | $ / shares |  |  | $ 18.10 |

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