Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Mar. 31, 2019	Dec. 31, 2018	Jan. 24, 2018
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	48.30%
DavidShield & PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%	25.00%	50.00%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%	50.00%	50.00%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	49.50%	49.50%
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)	18.40%	18.40%
Tuckerman Capital Fund III, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	16.00%	18.50%
White Mountains | DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%	25.00%