Investments in Unconsolidated Entities (Details) - White Mountains - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Investments in unconsolidated affiliates
Other Investments	$ 497.6	$ 325.6
Equity method eligible private equity securities, at fair value
Investments in unconsolidated affiliates
Other Investments	293.6	138.1
Investments, accounted for under the equity method
Investments in unconsolidated affiliates
Other Investments	1.0	1.3
Total investments in equity method eligible unconsolidated entities
Investments in unconsolidated affiliates
Other Investments	294.6	139.4
Other unconsolidated investments
Investments in unconsolidated affiliates
Other Investments	$ 203.0	$ 186.2