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Quarterly Financial Data (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Quarterly Financial Data [Abstract]                      
Operating revenues $ 5,704 $ 5,575 $ 5,742 $ 4,944 $ 5,258 $ 5,303 $ 5,731 $ 4,854      
Operating income 820 798 972 616 741 845 1,215 606 $ 3,206 $ 3,407 $ 3,522
Income before income taxes 817 786 960 602 736 832 1,165 532 3,164 3,265 3,450
Net income $ 654 $ 615 $ 733 $ 463 $ 1,747 [1] $ 528 $ 743 $ 339 $ 2,465 $ 3,357 $ 2,183
Basic (in dollars per share) $ 1.17 $ 1.08 $ 1.27 $ 0.79 $ 2.95 [1] $ 0.88 $ 1.23 $ 0.55 $ 4.30 $ 5.58 $ 3.48
Diluted (in dollars per share) $ 1.17 $ 1.08 $ 1.27 $ 0.79 $ 2.94 [1] $ 0.88 $ 1.23 $ 0.55 $ 4.29 $ 5.57 $ 3.45
Change in federal statutory rate                 $ 0 $ 1,270 $ 0
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent                 21.00%    
[1] Includes a $1.3 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.