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Investments in Unconsolidated Entities (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
White Mountains    
Investments in unconsolidated affiliates    
Total other long-term investments $ 289.7 $ 208.8
Equity method eligible private equity securities, at fair value    
Investments in unconsolidated affiliates    
Total other long-term investments 126.4 83.2
Equity method eligible private equity securities, at fair value | White Mountains    
Investments in unconsolidated affiliates    
Total other long-term investments 97.3 58.0
Investments, accounted for under the equity method | White Mountains    
Investments in unconsolidated affiliates    
Total other long-term investments 11.6 4.6
Total investments in equity method eligible unconsolidated entities | White Mountains    
Investments in unconsolidated affiliates    
Total other long-term investments 108.9 62.6
Other unconsolidated investments | White Mountains    
Investments in unconsolidated affiliates    
Total other long-term investments $ 180.8 $ 146.2