Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Sep. 30, 2018	Jan. 24, 2018	Dec. 31, 2017
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	50.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	0.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	49.50%		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)	22.10%		22.10%
Tuckerman Capital Fund III, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	21.80%		21.30%
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%