Statutory Capital and Surplus (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) yr	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2019
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	$ 15.4	$ 365.0	$ 404.6
Interest paid	8.8	1.4	2.1
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash	536.0
Other long-term investments	67.3
HG Re | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	698.9
Cash outside of collateral trusts	2.2
Collateral held in supplement trust	757.4
Cash and investments outside of collateral trusts	3.0
BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	413.7
Statutory net income (loss)	(34.6)	$ (25.4)	$ (32.7)
Surplus required	66.0
HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value	$ 619.0
Ownership interest (as a percent)	96.90%
Preferred stock dividend rate	6.00%
NSM Insurance HoldCo, LLC | NSM Insurance HoldCo, LLC
Statutory Accounting Practices [Line Items]
Cash	$ 16.2
MediaAlpha
Statutory Accounting Practices [Line Items]
Ownership interest (as a percent)	61.00%	64.40%
Cash	$ 5.7	$ 9.1
Distributions made	15.9
Unrestricted cash	5.7
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	17.7	4.0
Interest paid	$ 5.3	$ 1.0
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	4.60%	3.78%
Basis spread on variable rate	300.00%
Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Preferred stocks | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable	$ 288.1
Preferred stocks | White Mountains | HG Global | HG Global-BAM
Statutory Accounting Practices [Line Items]
Dividends payable	278.5
Common Equities | Other Operations
Statutory Accounting Practices [Line Items]
Distributions made	3.8
Common equity securities	$ 925.6
Combination One | Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate | yr		5
Combination One | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate	5
Combination Two | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate | yr	3
US Treasury Rate | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	4.60%
Fixed interest rate | Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)		8.00%
Fixed interest rate | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Forecast | Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				5.70%
Forecast | US Treasury Rate | Surplus Note | BAM | HG Global-BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				5.70%
Parent | MediaAlpha
Statutory Accounting Practices [Line Items]
Distributions made	$ 9.8