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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.