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Common Stock and Earnings Per Share - Summary of Common Stock (Details) - USD ($) shares in Thousands, $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Summary of Common Stock [Line Items] |  |  |  |
Balance, Shares | 49,100 | 45,900 |  |
Contributions to Pension, Equity | $ 0.0 | $ 19.5 | $ 0.0 |
Common Stock [Member] |  |  |  |
Summary of Common Stock [Line Items] |  |  |  |
Balance, Shares | 49,075 | 45,929 | 41,401 | 39,377
Balance, Equity | $ 1,271.4 | $ 1,107.6 | $ 885.2 | $ 784.7
Employee Stock Purchase Program, Shares | 18 | 18 | 16 |
Employee Stock Purchase Program, Equity | $ 0.9 | $ 0.8 | $ 0.7 |
Invest Direct, Shares | 383 | 378 | 395 |
Invest Direct, Equity | $ 19.0 | $ 18.9 | $ 18.5 |
Options and Stock Awards, Shares | 43 | 78 | 301 |
Options and Stock Awards, Equity | $ 8.6 | $ 8.0 | $ 17.9 |
Equity Issuance Program, Shares | 1,289 | 1,851 | 1,312 |
Equity Issuance Program, Equity | $ 69.9 | $ 90.0 | $ 63.4 |
Forward Sale Agreement and Issuance, Shares | 1,413 | 1,807 |  |
Forward Sales Agreement and Issuance, Equity | $ 65.4 | $ 85.2 |  |
Contributions to Pension, Shares | 0 | 396 | 0 |
Contributions to Pension, Equity |  | $ 19.5 |  |
Equity Issuance Program Shares Authorized | 13,600 |  |  |
Equity Issuance Program Shares Available for Issuance | 4,000 |  |  |

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