Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Jun. 30, 2019	Jun. 03, 2019	Apr. 04, 2019	Apr. 03, 2019	Dec. 31, 2018	Jan. 24, 2018
DavidShield & PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%				50.00%
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	48.30%
durchblicker
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	45.00%				45.00%
Elementum Holdings, L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	30.00%	30.00%
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)	16.20%				18.50%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)			99.10%	49.50%	49.50%
DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%				50.00%	50.00%
PassportCard
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%				25.00%	50.00%
White Mountains | DavidShield
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	25.00%				25.00%