Investments in Unconsolidated Entities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 253.1	$ 208.8
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Total other long-term investments	108.4	83.2
Equity method eligible private equity securities, at fair value | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	84.7	58.0
Investments, accounted for under the equity method | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	5.7	4.6
Total investments in equity method eligible unconsolidated entities | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	90.4	62.6
Other unconsolidated investments | White Mountains
Investments in unconsolidated affiliates
Total other long-term investments	$ 162.7	$ 146.2