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Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Quarterly Financial Data [Abstract] |  |  |  |  |  |  |  |  |  |  |  |
Operating revenues | $ 5,274 |  | $ 5,271 | $ 5,744 | $ 4,883 | $ 5,076 | $ 5,139 | $ 5,384 | $ 4,826 | $ 21,171 | $ 20,425 | $ 19,820
Operating income | 773 |  | 834 | 1,250 | 658 | 846 | 695 | 1,276 | 944 | 3,515 | 3,760 | 4,116
Income before income taxes | 737 |  | 791 | 1,170 | 553 | 809 | 618 | 1,304 | 816 | 3,251 | 3,547 | 3,479
Net income | $ 1,888 | [1] | $ 503 | $ 746 | $ 351 | $ 522 | $ 388 | $ 820 | $ 513 | $ 3,488 | $ 2,244 | $ 2,181
Basic (in dollars per share) | $ 3.19 | [1] | $ 0.84 | $ 1.24 | $ 0.57 | $ 0.85 | $ 0.63 | $ 1.30 | $ 0.80 | $ 5.80 | $ 3.58 | $ 3.30
Diluted (in dollars per share) | $ 3.18 | [1] | $ 0.84 | $ 1.23 | $ 0.57 | $ 0.84 | $ 0.62 | $ 1.28 | $ 0.79 | $ 5.79 | $ 3.55 | $ 3.27
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results |  |  |  |  |  |  |  |  |  | $ 544 | $ 544 | $ 427
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Basic Earnings Per Share |  |  |  |  |  |  |  |  |  | $ 0.82 | $ 0.47 | $ 0.34
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share |  |  |  |  |  |  |  |  |  | $ 0.82 | $ 0.46 | $ 0.34
Change in federal statutory rate |  |  |  |  |  |  |  |  |  | $ 1,410 | $ 0 | $ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent |  |  |  |  |  |  |  |  |  | 21.00% |  |
[1] | Includes a $1.4 billion reduction in Provision for income taxes related to the Tax Cuts and Jobs Act legislation enacted in December 2017, which resulted in a re-measurement of the Company's deferred tax assets and liabilities at the new federal corporate tax rate of 21 percent. See Note 14 to the Consolidated Financial Statements for further information.

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