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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%

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