Note 21: Unaudited Quarterly Data Presented in the tables below are supplemental, unaudited, consolidated, quarterly financial data for each of the four quarters in the years ended December 31, 2019 and 2018, respectively. The operating results for any quarter are not indicative of results that may be expected for a full year or any future periods. | | | | | | | | | | | | | | | | | | 2019 | | First Quarter | | Second Quarter | | Third Quarter | | Fourth Quarter | Operating revenues | $ | 813 |
| | $ | 882 |
| | $ | 1,013 |
| | $ | 902 |
| Operating income | 238 |
| | 302 |
| | 406 |
| | 224 |
| Net income attributable to common shareholders | 113 |
| | 170 |
| | 240 |
| | 98 |
| Basic earnings per share: (a) | | | | | | | | Net income attributable to common shareholders | $ | 0.62 |
| | $ | 0.94 |
| | $ | 1.33 |
| | $ | 0.54 |
| Diluted earnings per share: | | | | | | | | Net income attributable to common shareholders | 0.62 |
| | 0.94 |
| | 1.33 |
| | 0.54 |
|
| | (a) | Amounts may not sum due to rounding. |
| | | | | | | | | | | | | | | | | | 2018 | | First Quarter | | Second Quarter | | Third Quarter | | Fourth Quarter | Operating revenues | $ | 761 |
| | $ | 853 |
| | $ | 976 |
| | $ | 850 |
| Operating income | 217 |
| | 302 |
| | 335 |
| | 248 |
| Net income attributable to common shareholders | 106 |
| | 162 |
| | 187 |
| | 112 |
| Basic earnings per share: | | | | | | | | Net income attributable to common shareholders | $ | 0.60 |
| | $ | 0.90 |
| | $ | 1.04 |
| | $ | 0.62 |
| Diluted earnings per share: (a) | | | | | | | | Net income attributable to common shareholders | 0.59 |
| | 0.91 |
| | 1.04 |
| | 0.62 |
|
(a) Amounts may not sum due to rounding.
|