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Summary of Significant Accounting Policies (Noncontrolling Interest) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended |  |  |  |  |  |  |  |  | 12 Months Ended | 3 Months Ended | 12 Months Ended | 1 Months Ended |  | 12 Months Ended | 61 Months Ended | 1 Months Ended
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Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 White Mountains Consolidated Limited Partnership | Dec. 31, 2012 White Mountains Consolidated Limited Partnership | Dec. 31, 2013 A W G Dewar Inc | Dec. 31, 2012 A W G Dewar Inc | Sep. 30, 2013 Passage 2 Health Limited | Dec. 31, 2012 Passage 2 Health Limited | Dec. 31, 2013 OneBeacon Ltd. | Dec. 31, 2012 OneBeacon Ltd. | Dec. 31, 2012 White Mountains | Sep. 30, 2012 HG Global | Dec. 31, 2013 HG Global | May 31, 2007 Preferred stocks Sirius Internation Group Ltd Period quarter | May 24, 2007 Preferred stocks Sirius Internation Group Ltd | Dec. 31, 2012 BAM Management | Dec. 31, 2012 Common shares and paid-in surplus | Dec. 31, 2012 Non-controlling Interests | Dec. 31, 2013 BAM Non-controlling Interests | Dec. 31, 2012 BAM Non-controlling Interests | Dec. 31, 2013 Interest Rate Cap Sirius Internation Group Ltd | Jun. 30, 2022 Interest Rate Cap Sirius Internation Group Ltd | May 31, 2007 Interest Rate Cap Preferred stocks Sirius Internation Group Ltd | May 31, 2007 Scenario, 1 [Member] Preferred stocks Sirius Internation Group Ltd | May 31, 2007 Scenario, 1 [Member] Interest Rate Cap Preferred stocks Sirius Internation Group Ltd | May 31, 2007 Scenario, 2 [Member] Preferred stocks Sirius Internation Group Ltd
Noncontrolling Interest [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 25.00% |  | 24.80% | 24.80% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Capital contributions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 594.5 | $ 600.0 |  |  |  | $ 14.5 |  |  |  |  |  |  |  |  |  |  |  |
Fair Value of Net Assets Acquired |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.2 | (2.2) |  |  |  |  |  |  |  |  |  |
Pre-tax loss | 134.2 | 55.3 | 3.6 | 151.8 | (50.8) | 135.1 | 37.6 | 140.9 | 344.9 | 262.8 | 98.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (78.6) | [1] | (36.3) | [1] |  |  |  |  |  |
Non-cumulative perpetual preference shares issued |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 250,000,000 |  |  |  |  |  |  |  |  |  |  | 250,000,000 |  |  |
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investments recorded at fair value |  |  |  |  |  |  |  |  | 95.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from non-cumulative perpetual preference shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 245.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preference shares issuance cost and commission |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.3 |  |  |  |  |  |  |  |  |  |  |  |  |  |
years of interest rate cap |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5 years | 5 years |  |  |  |
Derivative, Cap Interest Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.30% |  |  |  |  |
Preferred Stock Redemption Basis Spread on Treasury Issue Rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Right to elect directors on board number |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period for non payment of dividend which give right to elect directors on board |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Minimum required payment period |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Preferred stock dividend rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  | 7.506% |  |  | 7.506%
Preference shares reference rate for dividend |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 month LIBOR |
Preference shares floating rate of dividend amount in excess of libor |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.20% |  |
Noncontrolling interest—consolidated limited partnerships |  |  |  |  |  |  |  |  |  |  |  | $ 46.1 | $ 41.5 | $ 3.1 | $ 2.8 |  | $ 0.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | BAM manages its affairs on a statutory accounting basis. BAM’s statutory surplus includes surplus notes and is not reduced by accruals of interest expense on the surplus notes. BAM’s statutory surplus is reduced only after a payment of principal or interest has been approved by the New York Department of Financial Services.

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