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Significant Transactions Significant Transactions (Acquisitions) (Details) ₪ in Millions, $ in Millions |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 06, 2015 USD ($) | Oct. 10, 2014 USD ($) | Sep. 30, 2015 USD ($) | Jan. 31, 2014 USD ($) | Feb. 23, 2015 ILS (₪) | Feb. 23, 2015 USD ($) | Feb. 19, 2014 ILS (₪) | Feb. 19, 2014 USD ($) | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($) | Jun. 30, 2015 USD ($) | Mar. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Sep. 30, 2014 USD ($) | Jun. 30, 2014 USD ($) | Mar. 31, 2014 USD ($) | Dec. 31, 2015 ILS (₪) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Dec. 31, 2013 USD ($) | Sep. 01, 2015 USD ($) | Mar. 14, 2014 USD ($) | Dec. 31, 2012 USD ($)
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets |  |  |  |  |  |  |  |  | $ 10,284.5 |  |  |  | $ 10,455.7 |  |  |  |  | $ 10,284.5 | $ 10,455.7 |  |  |  |
Liabilities |  |  |  |  |  |  |  |  | 5,916.5 |  |  |  | 5,917.3 |  |  |  |  | 5,916.5 | 5,917.3 |  |  |  |
White Mountains’s common shareholders’ equity, including portion attributable to NCI |  |  |  |  |  |  |  |  | 4,368.0 |  |  |  | 4,538.4 |  |  |  |  | 4,368.0 | 4,538.4 |  |  |  |
Other assets |  |  |  |  |  |  |  |  | 318.7 |  |  |  | 348.7 |  |  |  |  | 318.7 | 348.7 |  |  |  |
Goodwill |  |  |  |  |  |  |  |  | 187.9 |  |  |  | 168.9 |  |  |  |  | 187.9 | 168.9 | $ 0.0 |  |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  |  |  |  | 187.8 |  |  |  | 182.3 |  |  |  |  | 187.8 | 182.3 | 5.1 |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 37.1 | 190.1 | 0.0 |  |  |
Non-controlling interest in consolidated subsidiaries |  |  |  |  |  |  |  |  | 6.1 | $ (16.0) | $ (0.9) | $ (7.3) | (15.0) | $ (11.2) | $ 4.6 | $ (0.6) |  | (18.1) | (22.2) | $ (12.5) |  |  |
Cashboard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities |  |  |  |  |  | $ 1.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  | 56.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | $ 0.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cancerinsurance.com [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates | $ 3.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | 1.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities | $ 2.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Combination, Contingent Consideration, Liability |  |  |  |  |  |  |  |  | 6.8 |  |  |  |  |  |  |  |  | 6.8 |  |  |  |  |
Tranzact [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  | $ 177.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets |  | 332.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities |  | $ 108.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  | 63.20% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Distributed Earnings |  | $ 44.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
White Mountains’s common shareholders’ equity, including portion attributable to NCI |  | 281.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  | 145.1 |  |  |  |  |  |  | 163.8 |  |  |  | 145.1 |  |  |  |  | 163.8 | 145.1 |  |  |  |
Intangible Assets, Net (Excluding Goodwill) |  | 146.3 |  |  |  |  |  |  | 156.1 |  |  |  | 142.8 |  |  |  |  | 156.1 | 142.8 |  |  |  |
Business Combination, Contingent Consideration, Liability |  | 7.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tranzutary [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  |  |  |  |  |  |  |  |  |  | $ 12.0 |  |  |  |  |  |  |  |  |  |  |
Assets |  | 29.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.1 |  |  |  |  |  | $ 4.1 |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  | 36.00% |  |  |  |  |  | 36.00% |  |  |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  |  |  |  |  |  |  |  | $ 28.9 |  |  |  |  |  | $ 28.9 |  |  |  |
TruBridge [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  | $ 31.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 54.5 |  |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.3 |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |
Goodwill |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18.7 |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 28.1 |  |
Business Combination, Contingent Consideration, Liability |  |  |  |  |  |  |  |  | 9.7 |  |  |  |  |  |  |  |  | 9.7 |  |  |  |  |
MediaAlpha |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Interest in Subsidiaries and Affiliates |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 28.1 |  |  |  |  |  |  |
Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 70.1 |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 10.0 |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.00% |
Goodwill |  |  |  |  |  |  |  |  | 18.3 |  |  |  | 18.3 |  |  |  |  | 18.3 | 18.3 |  |  | $ 18.3 |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  |  |  |  | 24.4 |  |  |  | 32.5 |  |  |  |  | $ 24.4 | 32.5 |  |  | $ 38.5 |
Percent of gross profit |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 62.50% | 62.50% |  |  |  |  |
Business Combination, Contingent Consideration, Liability |  |  |  |  |  |  |  |  | $ 7.8 |  |  |  | $ 7.9 |  |  |  |  | $ 7.8 | $ 7.9 |  |  |  |
WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Liabilities |  |  |  |  |  |  |  | $ 0.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  |  | 96.10% |  |  |  | 63.30% |  |  |  |  | 96.10% | 63.30% |  |  |  |
Goodwill |  |  |  |  |  |  |  | $ 5.5 | $ 5.8 |  |  |  | $ 5.5 |  |  |  |  | $ 5.8 | $ 5.5 |  |  |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  |  |  |  | 4.5 |  |  |  | 2.5 |  |  |  |  | 4.5 | 2.5 |  |  |  |
Star & Shield LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  | $ 1.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase of Surplus Notes |  |  |  |  |  |  |  |  | 4.0 |  |  |  | 17.0 |  |  |  |  | 4.0 | 17.0 |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  | $ 21.0 |  |  |  | $ 17.0 |  |  |  |  | 21.0 | $ 17.0 |  |  |  |
HG Global |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Surplus notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 503.0
Total | WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership interest (as a percent) |  |  |  |  |  |  |  | 54.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Israel, New Shekels | Preferred stocks | WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates | ₪ |  |  |  |  |  |  | ₪ 31.5 |  |  |  |  |  |  |  |  |  | ₪ 79.6 |  |  |  |  |  |
Israel, New Shekels | Total | Cashboard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates | ₪ |  |  |  |  | ₪ 9.5 |  |  |  |  |  |  |  |  |  |  |  | 26.4 |  |  |  |  |  |
Israel, New Shekels | Total | WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates | ₪ |  |  |  |  |  |  | ₪ 14.4 |  |  |  |  |  |  |  |  |  | ₪ 11.8 |  |  |  |  |  |
United States of America, Dollars | Preferred stocks | WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  |  |  |  | $ 9.0 |  |  |  |  |  |  |  |  |  | 20.7 |  |  |  |  |
United States of America, Dollars | Total | Cashboard [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  |  | 2.4 |  |  |  |  |  |  |  |  |  |  |  | 6.5 |  |  |  |  |
United States of America, Dollars | Total | WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Businesses and Interest in Affiliates |  |  |  |  |  |  |  | 4.1 |  |  |  |  |  |  |  |  |  | $ 3.1 |  |  |  |  |
WOBI [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Assets |  |  |  |  |  | 5.5 |  | 13.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible Assets, Net (Excluding Goodwill) |  |  |  |  |  | $ 2.8 |  | $ 2.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Tangible Assets [Member] | Tranzact [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unusual or Infrequent Item [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Other assets |  | $ 41.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

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