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Common Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Aug. 26, 2010 | Nov. 17, 2006
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Authorized repurchase of common shares | 2,600,000 |  |  |  | 1,000,000
Additional authorized repurchase of common shares | 1,000,000 |  |  | 600,000 |
Shares remaining under share repurchase authorization | 685,496,000 |  |  |  |
Common shares repurchased and retired during the period | 1,329,640 | 646,502 | 687,871 |  |
Number of Modified Dutch Auction | 2 |  |  |  |
Value of common shares repurchased and retired during the period | $ 669.1 | $ 253.0 | $ 225.5 |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 503 | $ 390 | $ 328 |  |
Common Shares Issued [Abstract] |  |  |  |  |
Common shares issued, total (in shares) | 44,054 | 29,432 | 23,050 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period |  |  | 2,400 |  |
Shares issued in satisfaction of options exercised (in shares) | 32,160 | 27,250 | 19,750 |  |
Shares issued to directors (in shares) | 1,617 | 2,182 | 900 |  |
Dividends, Common Stock [Abstract] |  |  |  |  |
Cash dividends declared and paid | 6.6 | 8.0 | 8.8 |  |
Dividends delared and paid (in dollars per share) |  | $ 1 | $ 1 |  |
General Board Authorization |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Common shares repurchased and retired during the period | 502,801 | 313,967 | 677,125 |  |
Value of common shares repurchased and retired during the period | 256.0 | 114.0 | 222.0 |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 508 | $ 364 | $ 264 |  |
Tender Offer |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Common shares repurchased and retired during the period | 816,829 | 332,346 |  |  |
Value of common shares repurchased and retired during the period | 408.6 | 138.8 |  |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 500 | $ 418 |  |  |
Shares Repurchased For Employee Benefit Plan [Member] |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Common shares repurchased and retired during the period | 10,010 | 189 | 10,746 |  |
Non-controlling Interests |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | 0 | 0 | 0 |  |
Total Equity includes Noncontrolling Interest [Member] |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | 669.1 | 253.0 | 225.6 |  |
White Mountains | Common shares and paid-in surplus |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | 221.3 | 107.2 | 112.4 |  |
White Mountains | Retained earnings |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | 447.8 | 145.8 | 113.2 |  |
White Mountains | AOCI, after-tax |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | 0 | 0 | 0 |  |
White Mountains | Common shareholders’ equity |  |  |  |  |
Common Shares Repurchased and Retired [Abstract] |  |  |  |  |
Value of common shares repurchased and retired during the period | $ 669.1 | $ 253.0 | $ 225.6 |  |

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