Investments in Unconsolidated Affiliates (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
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