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Common Equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Equity [Abstract] |
Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] | Basic and diluted earnings per share for the Company were calculated as follows:
(In millions except per share data) | 2017 | 2016 | 2015
Net income | $ | 619.0 |  | $ | 338.2 |  | $ | 271.3 |
Average common shares outstanding: |  |  |
Basic average common shares outstanding | 199.7 |  | 199.7 |  | 199.6 |
Effect of dilutive securities: |  |  |
Contingently issuable shares (performance and restricted stock units) | 0.3 |  | 0.2 |  | — |
Diluted average common shares outstanding | 200.0 |  | 199.9 |  | 199.6 |
Basic earnings per average common share | $ | 3.10 |  | $ | 1.69 |  | $ | 1.36 |
Diluted earnings per average common share | $ | 3.10 |  | $ | 1.69 |  | $ | 1.36 |
Anti-dilutive shares excluded from earnings per share calculation | — |  | — |  | — |

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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 + Details
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