Investments in Unconsolidated Entities - White Mountain's Investments (Details)	Sep. 30, 2019	May 31, 2019	Dec. 31, 2018
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%
JAM Partners L.P.
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	11.10%		12.30%
Kudu
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)			49.50%
Minimum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	18.80%		18.50%
Minimum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	10.00%		10.30%
Minimum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	3.20%
Maximum | Tuckerman Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	62.40%		62.30%
Maximum | Enlightenment Capital Funds
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	36.50%		36.50%
Maximum | Kudu Investments
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	30.00%