Statutory Capital and Surplus (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) yr	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2018
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	$ 365,000,000	$ 404,600,000	$ 76,100,000
Interest paid	1,400,000	2,100,000	100,000
Distributions made	4,600,000
Common equity securities	2,129,700,000
Other Operations
Statutory Accounting Practices [Line Items]
Unrestricted cash	1,682,600,000
Other long-term investments	62,000,000
HG Re | HG Global BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	677,000,000
Surplus required	1
Collateral held in supplement trust	715,100,000
Cash and investments outside of collateral trusts	1,000,000
BAM | HG Global BAM
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	427,300,000
Statutory net income (loss)	25,400,000	$ 32,700,000	$ 32,000,000
Surplus required	66,000,000
HG Global | HG Global BAM
Statutory Accounting Practices [Line Items]
Preferred stock, value	$ 619,000,000
Ownership interest (as a percent)	96.90%
Preferred stock dividend rate	6.00%
White Mountains | MediaAlpha
Statutory Accounting Practices [Line Items]
Distributions made	$ 3,300,000
MediaAlpha
Statutory Accounting Practices [Line Items]
Ownership interest (as a percent)	64.40%	60.00%
Distributions made	$ 5,200,000
Unrestricted cash	9,100,000
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Repayments of debt, principal	4,000,000
Interest paid	$ 1,000,000
Surplus Note | BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	3.78%	3.54%
Debt instrument, basis spread on variable rate	30000.00%
Surplus Note | BAM | HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)	8.00%
Debt instrument, basis spread on variable rate		3.00%
Preferred stocks | HG Global | HG Global BAM
Statutory Accounting Practices [Line Items]
Dividends payable	$ 235,600,000
Preferred stocks | White Mountains | HG Global | HG Global BAM
Statutory Accounting Practices [Line Items]
Dividends payable	227,900,000
Common Equities | Other Operations
Statutory Accounting Practices [Line Items]
Common equity securities	$ 866,100,000
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
Statutory Accounting Practices [Line Items]
Number of years of change in interest rate | yr	3
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)	3.78%
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)				4.60%
Forecast | US Treasury Rate | Surplus Note | BAM | HG Global BAM
Statutory Accounting Practices [Line Items]
Interest rate (as a percent)				4.60%