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Quarterly Financial Data | 12 Months Ended
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Dec. 31, 2017
Quarterly Financial Data [Abstract] |
Quarterly Financial Information [Text Block] | Quarterly Financial Data (Unaudited) Due to the seasonal fluctuations and other factors of the Company's businesses, the operating results for interim periods are not necessarily indicative of the results that may be expected for the year. 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 The entire disclosure for quarterly financial data. Includes, but is not limited to, tabular presentation of financial information for fiscal quarters, effect of year-end adjustments, and an explanation of matters or transactions that affect comparability of the information. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 270 -URI http://asc.fasb.org/topic&trid=2126967 + Details
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