Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Jun. 30, 2018	Dec. 31, 2017
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	0.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	0.00%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%	45.00%
Compare.com
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	22.00%	22.00%
OneTitle
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	20.00%	20.00%
Tuckerman Capital Fund III
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	20.00%	21.00%
White Mountains | PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%
DavidShield | PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%