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Investments in Unconsolidated Affiliates (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Equity method eligible private equity securities, at fair value |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | $ 83.2 | $ 88.2
White Mountains |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | 208.8 | 172.8
White Mountains | Equity method eligible private equity securities, at fair value |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | 58.0 | 55.6
White Mountains | Investments, accounted for under the equity method |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | 4.6 | 3.5
White Mountains | Total investments in equity method eligible unconsolidated entities |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | 62.6 | 59.1
White Mountains | Other Investments |  |
Investments in and Advances to Affiliates [Line Items] |  |
Total other long-term investments | $ 146.2 | $ 113.7

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