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Statutory Capital and Surplus (Details) | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 USD ($) | Dec. 31, 2015 USD ($) yr | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Dec. 31, 2016 | Dec. 31, 2012 USD ($)
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | $ 3,995,700,000 | $ 3,913,200,000 | $ 3,995,700,000 |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | $ 48,000,000 |  |  |  |
Equity held in insurance and reinsurance subsidiaries |  | 505,000,000 |  |  |  |
Distributions made |  | $ 6,000,000 |  |  |  |
Payments for Repurchase of Common Stock |  | 268,600,000 | 128,200,000 | $ 79,800,000 |  |
Fair value investments | 3,343,600,000 | 4,050,900,000 | 3,343,600,000 | 3,889,000,000 |  |
Trading Securities |  | 2,639,700,000 |  |  |  |
Dividend Capacity |  |  |  |  |  |
Other long-term investments | 331,900,000 | 315,800,000 | 331,900,000 |  |  |
OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory net income (loss) |  | 44,500,000 | (14,200,000) | 100,100,000 |  |
Stockholders' Equity Attributable to Parent | 787,500,000 | 755,200,000 | 787,500,000 |  |  |
Distributions made |  | 80,000,000 |  | 80,200,000 |  |
Unrestricted cash |  | 65,300,000 |  |  |  |
Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | 1,425,500,000 | 1,344,300,000 | 1,425,500,000 |  |  |
Unrestricted cash |  | 73,700,000 |  |  |  |
Dividend Capacity |  |  |  |  |  |
Other long-term investments |  | 17,600,000 |  |  |  |
Unregulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made |  | 5,300,000 | 4,800,000 |  |  |
Surplus notes |  | 101,000,000 |  |  |  |
Unrestricted cash |  | 104,400,000 |  |  |  |
Lone Tree Insurance Group Ltd |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Noncash distribution received |  |  |  | 27,900,000 |  |
Proceeds from Contributions from Affiliates |  | 15.0 |  |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Surplus notes | 17,000,000 | 21,000,000 | 17,000,000 |  |  |
Star & Shield Insurance Exchange [Member] | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus | 6,500,000 | 5,000,000 | 6,500,000 |  |  |
Statutory net income (loss) |  | 3,500,000 | 9,500,000 |  |  |
Surplus required |  | 1,500,000 |  |  |  |
OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Dividends available, net of noncontrolling interest |  | 180,000,000 |  |  |  |
HG Global |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Surplus notes |  |  |  |  |  | $ 503,000,000
HG Global | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Preferred stock, value |  | $ 613,000,000 |  |  |  |
Ownership interest (as a percent) |  | 96.90% |  |  |  |
Preferred stock dividend rate |  | 6.00% |  |  |  |
HG Re | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | $ 460,800,000 |  |  |  |
Collateral Held in Supplement Trust |  | 401,900,000 |  |  |  |
Dividend Capacity |  |  |  |  |  |
Accrued interest on surplus notes |  | $ 53,800,000 |  |  |  |
BAM | HG Global-BAM |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | 437,300,000 |  |  |  |
Statutory net income (loss) |  | 32,000,000 | 31,800,000 | 29,300,000 |  |
Surplus required |  | 66,000,000 |  |  |  |
Subsidiaries |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent |  | 554,000,000 |  |  |  |
One Beacon Insurance Company | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made | 700,500,000 |  |  |  |  |
Noncash distribution received | 151,200,000 |  |  |  |  |
Split Rock [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus | 122,600,000 | 249,500,000 | 122,600,000 |  |  |
Statutory net income (loss) |  | 45,500,000 | 46,200,000 | $ (38,700,000) |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 37,400,000 |  |  |  |
Payments of Distributions to Affiliates |  | 85,000,000 |  |  |  |
White Mountains | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Distributions made |  | 60,300,000 |  |  |  |
White Mountains | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Unrestricted cash |  | 14,400,000 |  |  |  |
Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Surplus required |  | 117,700,000 |  |  |  |
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus |  | 622,300,000 |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 26,600,000 |  |  |  |
Distributions made |  | 44,900,000 | 10,000,000 |  |  |
Payments for Repurchase of Common Stock |  | 66,000,000 |  |  |  |
Payments of Capital Distribution |  |  | 10,000,000 |  |  |
Statutory accounting practices earned surplus |  | 68,300,000 |  |  |  |
Atlantic Speciality Insurance Company (ASIC) [Member] | Regulatory Approval [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Amount available for dividend distributions without prior approval from regulatory authorities |  | 62,200,000 |  |  |  |
White Mountains Life Re | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus | $ 76,000,000 | 77,500,000 | $ 76,000,000 |  |  |
Surplus required |  | $ 500,000 |  |  |  |
Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Interest rate (as a percent) |  | 8.00% |  |  |  |
basis points |  | 3.00% |  |  |  |
Preferred stocks | HG Global | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Dividends Payable |  | $ 139,700,000 |  |  |  |
Preferred stocks | White Mountains | HG Global | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Dividends Payable |  | $ 135,300,000 |  |  |  |
SEK |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Exchange rate | 7.7737 | 8.4247 | 7.7737 | 6.4339 |  |
Common Stock [Member] | OneBeacon |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Other long-term investments |  | $ 51,500,000 |  |  |  |
Common Equities [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Trading Securities |  | 54,600,000 |  |  |  |
Common Equities [Member] | Other Operations |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Trading Securities |  | 292,300,000 |  |  |  |
Retained earnings |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Stockholders' Equity Attributable to Parent | $ 3,010,500,000 | 3,084,900,000 | $ 3,010,500,000 | $ 2,801,900,000 |  | $ 2,542,500,000
Amount available for dividend distributions without prior approval from regulatory authorities |  | $ 2,600,000,000 |  |  |  |
Retained earnings | Split Rock [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory Accounting Practices, Statutory Capital, Percent |  | 15.00% |  |  |  |
Retained earnings | Atlantic Speciality Insurance Company (ASIC) [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory Accounting Practices, Statutory Capital, Percent |  | 10.00% |  |  |  |
U.S and Bermuda [Member] | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Statutory capital and surplus | $ 721,500,000 | $ 622,300,000 | $ 721,500,000 |  |  |
Surplus Note | Unregulated Operation | OneBeacon |  |  |  |  |  |
Statutory Accounting Practices [Line Items] |  |  |  |  |  |
Fair value investments |  | $ 51,500,000 |  |  |  |
Combination One [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Number of years of change in interest rate |  | 5 |  |  |  |
Combination Two [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Number of years of change in interest rate | yr |  | 3 |  |  |  |
US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Interest rate (as a percent) |  | 3.15% |  |  |  |
Fixed interest rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Interest rate (as a percent) |  | 8.00% |  |  |  |
Scenario, Forecast [Member] | US Treasury Rate [Member] | Surplus Note | BAM | HG Global-BAM |  |  |  |  |  |
Dividend Capacity |  |  |  |  |  |
Interest rate (as a percent) |  |  |  |  | 3.54% |

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