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Common Stock and Earnings Per Share - Summary of Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended |
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Sep. 30, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2012 Common Stock [Member] | Dec. 31, 2011 Common Stock [Member] | Dec. 31, 2010 Common Stock [Member] | Dec. 31, 2009 Common Stock [Member]
Class of Stock [Line Items] |  |  |  |  |  |  |  |
Common Stock Outstanding, Shares |  | 39,400,000 | 37,500,000 |  | 39,377,000 | 37,513,000 | 35,817,000 | 35,221,000
Common Stock Outstanding, Value |  |  |  |  | $ 784.7 | $ 705.6 | $ 636.1 | $ 613.4
Employee Stock Purchase Program, Shares |  |  |  |  | 20,000 | 20,000 | 19,000 |
Employee Stock Purchase Program, Value |  |  |  |  | 0.8 | 0.8 | 0.6 |
Invest Direct, Shares |  |  |  |  | 474,000 | 437,000 | 346,000 |
Invest Direct, Value |  |  |  |  | 19.2 | 17.2 | 11.7 |
Options and Stock Awards, Shares |  |  |  |  | 95,000 | 109,000 | 51,000 |
Options and Stock Awards, Value |  |  |  |  | 6.0 | 6.7 | 4.4 |
Equity Issuance Program, Shares |  |  |  |  | 1,275,000 | 400,000 | 180,000 |
Equity Issuance Program, Value |  |  |  |  | 53.1 | 16.0 | 6.0 |
Purchase of Non-Controlling Interest, Shares | 200,000 |  |  |  |  | 222,000 |  |
Purchase of Non-Controlling Interest, Value |  |  |  |  |  | 8.8 |  |
Contributions to Pension, Shares |  |  |  |  |  | 508,000 |  |
Contributions to Pension, Value |  | $ 0 | $ (20.0) | $ 0 |  | $ 20.0 |  |
Antidilutive Options to Purchase Shares of Common Stock Excluded from the Computation of Earnings Per Share |  |  |  |  | 200,000 | 300,000 | 500,000 |
Equity Issuance Program Shares Authorized |  |  |  |  | 9,600,000 |  |  |
Equity Issuance Program Shares Available for Issuance |  |  |  |  | 4,500,000 |  |  |

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