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Common Shareholders' Equity (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 62 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2011 | Aug. 26, 2010 | Nov. 17, 2006
Common Shares Repurchased and Retired |  |  |  |  |  |
Authorized repurchase of common shares (in shares) |  |  |  |  |  | 1,000,000
Additional authorized repurchase of common shares (in shares) |  |  |  |  | 600,000 |
Shares remaining under share repurchase authorization | 188,297 |  |  | 188,297 |  |
Common shares repurchased and retired during the period (in shares) | 313,967 | 677,125 |  | 1,411,703 |  |
Value of common shares repurchased and retired during the period | $ 114.2 | $ 222.0 |  | $ 537.5 |  |
Shares repurchased by the entity that were not subject to the Share Repurchase Plan (in shares) | 189 | 10,186 | 1,113 |  |  |
Common Shares Issued |  |  |  |  |  |
Common shares issued, total (in shares) | 29,432 | 23,050 | 52,420 |  |  |
Shares issued in satisfaction of options exercised (in shares) | 0 | 2,400 | 3,600 |  |  |
Restricted shares issued to key management personnel (in shares) | 27,250 | 19,750 | 47,820 |  |  |
Shares issued to directors (in shares) | 2,182 | 900 | 1,000 |  |  |
Dividends on Common Shares |  |  |  |  |  |
Cash dividends declared and paid | 8.0 | 8.8 | 8.9 |  |  |
Dividends declared and paid (in dollars per share) | $ 1.00 | $ 1.00 | $ 1.00 |  |  |
Tender Offer |  |  |  |  |  |
Common Shares Repurchased and Retired |  |  |  |  |  |
Number of modified Dutch auction | 2 |  |  |  |  |
Common shares repurchased and retired during the period (in shares) | 332,346 |  |  |  |  |
Value of each common share repurchased and retired during the period (in dollars per share) | $ 417.55 |  |  |  |  |
Value of common shares repurchased and retired during the period | $ 138.8 |  |  |  |  |

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