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Significant Transactions (Dispositions) (Details) - USD ($) $ / shares in Units, $ in Millions | 2 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2016 | Dec. 31, 2014 | Mar. 31, 2013 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Jul. 24, 2015 | Dec. 23, 2014
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  | $ 18.2 | $ (1.6) | $ 46.6 |  |
Gain (Loss) on Disposition of Stock in Subsidiary |  |  |  | 16.1 | 0.0 | 21.3 |  |
Premiums Written, Net |  |  |  | $ 1,172.6 | $ 1,239.0 | $ 1,102.2 |  |
Dividends declared and paid per White Mountains’s common share |  |  |  | $ 1.00 | $ 1.00 | $ 1.00 |  |
Total investments |  | $ 3,742.4 |  | $ 4,280.7 | $ 3,742.4 |  |  |
Symetra |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Sale of Stock, Price Per Share |  |  |  | $ 32.00 |  |  |  |
Dividends declared and paid per White Mountains’s common share |  |  |  | $ 0.50 |  |  |  |
Sirius Group |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Percentage of common shareholder's equity |  |  |  |  |  |  | 127.30% |
Additional cash that will be paid as part of the purchase price |  |  |  |  |  |  | $ 10.0 |
Total investments |  |  |  | $ 686.2 |  |  |  |
OneBeacon Runoff |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  | $ 101.0
Essentia | OneBeacon |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary |  |  | $ 23.0 |  |  |  |  |
Gain (Loss) on Disposition of Stock in Subsidiary, After Tax |  |  | $ 15.0 |  |  |  |  |
Gain (Loss) from Adjustments to Prior Period Extraordinary Items |  |  |  | 3.7 |  |  |  |
Fair Value [Member] | Other long-term investments | OneBeacon Runoff |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  | $ 64.9
Discontinued Operations |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  |  |  | $ 46.6 |  |
Discontinued Operations | Sirius Group |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  |  |  | 0.0 |  |
Total investments |  | 3,142.7 |  | 2,972.6 | 3,142.7 |  |  |
Discontinued Operations | OneBeacon Runoff |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  | 0.3 | $ (18.8) | $ 46.6 |  |
Surplus Note | Discontinued Operations | OneBeacon Runoff |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  | $ 36.1 |  |  |  |  |  |
Crop |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Proceeds related from termination of agreement |  |  |  | $ 3.0 |  |  |  |
Sale of Sirius | UNITED STATES |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Industry Loss Warranty Limit on Contract | $ 45.0 |  |  |  |  |  |  |
Industry Loss Trigger on Industry Loss Warranty | 15,000.0 |  |  |  |  |  |  |
Sale of Sirius | JAPAN |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Industry Loss Warranty Limit on Contract | 25.0 |  |  |  |  |  |  |
Industry Loss Trigger on Industry Loss Warranty | 12,500.0 |  |  |  |  |  |  |
Combination One [Member] | Sale of Sirius | UNITED STATES |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Industry Loss Warranty Limit on Contract | 75.0 |  |  |  |  |  |  |
Industry Loss Trigger on Industry Loss Warranty | 40,000.0 |  |  |  |  |  |  |
Combination Two [Member] | Sale of Sirius | UNITED STATES |  |  |  |  |  |  |  |
Significant Transactions |  |  |  |  |  |  |  |
Industry Loss Warranty Limit on Contract | 22.5 |  |  |  |  |  |  |
Industry Loss Trigger on Industry Loss Warranty | $ 50,000.0 |  |  |  |  |  |  |

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