Investments in Unconsolidated Affiliates Investments in Unconsolidated Affiliates (White Mountain's Investments) (Details)	Dec. 31, 2017
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	22.00%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%
OneTitle
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	20.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%
Tuckerman Capital Fund III
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	21.00%