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Quarterly Financial Data (Tables) | 12 Months Ended
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Dec. 31, 2017
Quarterly Financial Data [Abstract] |
Schedule of Quarterly Financial Information [Table Text Block] | In the Company's opinion, the following quarterly financial data includes all adjustments, consisting of normal recurring adjustments, necessary to fairly present such amounts. Summarized consolidated quarterly unaudited financial data is as follows:
Quarter Ended (In millions, except per share data) |  | March 31 | June 30 | September 30 | December 31 | Total
Operating revenues | 2017 | $ | 456.0 |  | $ | 586.4 |  | $ | 716.8 |  | $ | 501.9 |  | $ | 2,261.1 |
| 2016 | $ | 433.1 |  | $ | 551.4 |  | $ | 743.9 |  | $ | 530.8 |  | $ | 2,259.2 |
Operating income | 2017 | $ | 43.8 |  | $ | 143.5 |  | $ | 243.3 |  | $ | 79.7 |  | $ | 510.3 |
| 2016 | $ | 37.9 |  | $ | 125.9 |  | $ | 257.3 |  | $ | 82.2 |  | $ | 503.3 |
Net income | 2017 | $ | 36.0 |  | $ | 104.8 |  | $ | 183.4 |  | $ | 294.8 |  | $ | 619.0 |
| 2016 | $ | 25.2 |  | $ | 71.5 |  | $ | 183.6 |  | $ | 57.9 |  | $ | 338.2 |
Basic earnings per average common share (A) | 2017 | $ | 0.18 |  | $ | 0.52 |  | $ | 0.92 |  | $ | 1.48 |  | $ | 3.10 |
| 2016 | $ | 0.13 |  | $ | 0.35 |  | $ | 0.92 |  | $ | 0.29 |  | $ | 1.69 |
Diluted earnings per average common share (A) | 2017 | $ | 0.18 |  | $ | 0.52 |  | $ | 0.92 |  | $ | 1.48 |  | $ | 3.10 |
| 2016 | $ | 0.13 |  | $ | 0.35 |  | $ | 0.92 |  | $ | 0.29 |  | $ | 1.69 |
(A) | Due to the impact of dilution on the earnings per share calculation, quarterly earnings per share amounts may not add to the total.

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- Definition Tabular disclosure of quarterly financial data. Includes, but is not limited to, financial information for fiscal quarters, cumulative effect of a change in accounting principle and earnings per share data. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a)-(j) -URI http://asc.fasb.org/extlink&oid=84173487&loc=d3e1280-108306 + Details
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