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Common Shareholders’ Equity and Non-controlling Interests (Details) - USD ($) $ / shares in Units, $ in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
May 31, 2007 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2012
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period |  | $ 887.2 | $ 284.2 | $ 134.5 |
Value of each common share repurchased and retired during the period (in dollars per share) |  | $ 802 | $ 733 | $ 617 |
Common Shares Issued [Abstract] |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 47,030 | 25,016 | 28,405 |
Shares issued to directors (in shares) |  | 1,735 | 1,376 | 1,395 |
Dividends, Common Stock [Abstract] |  |  |  |  |
Cash dividends declared and paid |  | $ 5.4 | $ 6.0 | $ 6.2 |
Dividends delared and paid (in dollars per share) |  | $ 1.00 | $ 1.00 | $ 1.00 |
Non-controlling interests |  | $ 133.9 | $ 454.8 |  |
Non-controlling Interests |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period |  | 0.0 | 0.0 | $ 0.0 |
Total Non-Controlling Excluding Reciprocals [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Non-controlling interests |  | $ 35.8 | $ 115.2 |  |
Restricted Stock |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 25,365 | 23,640 | 23,440 |
Performance Shares |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 5,000 |  | 3,570 |
Common Stock |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Common shares repurchased and retired during the period |  | 1,106,145 | 387,495 | 217,879 |
General Board Authorization [Member] |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Additional authorized repurchase of common shares |  | 878,130 |  |  |
Common shares repurchased and retired during the period |  | 1,098,123 | 361,839 | 207,404 |
Value of common shares repurchased and retired during the period |  | $ 881.4 | $ 267.4 | $ 128.2 |
Value of each common share repurchased and retired during the period (in dollars per share) |  | $ 803 | $ 739 | $ 618 |
Chief Executive Officer |  |  |  |  |
Common Shares Issued [Abstract] |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 14,930 |  |  |
Repurchased for Employee Benefit Plans [Member] |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Common shares repurchased and retired during the period |  | 8,022 | 10,802 | 10,475 |
White Mountains |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Capital contributions |  |  |  |  | $ 594.5
Sirius Internation Group Ltd | Preferred stocks |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Preference shares floating rate of dividend amount in excess of libor | 3.20% |  |  |  |
Preferred Stock Redemption Basis Spread on Treasury Issue Rate | 45.00% |  |  |  |
OneBeacon Ltd. |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 23.90% | 24.50% |  |
Non-controlling interests |  | $ 244.6 | $ 245.6 |  |
HG/BAM [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 100.00% | 100.00% |  |
Non-controlling interests |  | $ (150.9) | $ (140.0) |  |
Star & Shield Insurance Exchange [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 100.00% | 100.00% |  |
Non-controlling interests |  | $ 4.4 | $ (16.0) |  |
Mutual and Reciprocals [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Non-controlling interests |  | $ (146.5) | $ (156.0) |  |
HG Global |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 3.10% | 3.10% |  |
Non-controlling interests |  | $ 16.6 | $ 17.1 |  |
MediaAlpha |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 40.00% | 40.00% |  |
Non-controlling interests |  | $ 11.7 | $ 14.4 |  |
Tranzact |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  |  | 36.80% |  |
Non-controlling interests |  |  | $ 79.4 |  |
WOBI [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 5.00% | 3.90% |  |
Non-controlling interests |  | $ 0.7 | $ 0.6 |  |
Buzzmove [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 29.10% | 0.00% |  |
Non-controlling interests |  | $ 2.9 | $ 0.0 |  |
A W G Dewar Inc [Member] |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 18.80% | 19.00% |  |
Non-controlling interests |  | $ 3.9 | $ 3.7 |  |
SIG Preference Shares |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners |  | 0.00% | 100.00% |  |
Non-controlling interests |  | $ 0.0 | $ 250.0 |  |
BAM Management |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Capital contributions |  |  |  |  | $ 14.5
OneBeacon |  |  |  |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Non-controlling interests |  | $ 244.6 | $ 249.3 |  |

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