Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 1,403	$ 1,555	$ 1,570	$ 1,391	$ 1,554	$ 1,593	$ 1,509	$ 1,310	$ 5,919	$ 5,966	$ 10,712
Weighted-Average Number Of Shares Outstanding
Basic									703.5	750.9	798.4
Dilutive effect of stock options									1.2	1.9	1.8
Dilutive effect of retention shares and units									1.4	1.5	1.5
Diluted									706.1	754.3	801.7
Earnings per share – basic	$ 2.03	$ 2.22	$ 2.23	$ 1.94	$ 2.13	$ 2.16	$ 1.98	$ 1.69	$ 8.41	$ 7.95	$ 13.42
Earnings per share – diluted	$ 2.02	$ 2.22	$ 2.22	$ 1.93	$ 2.12	$ 2.15	$ 1.98	$ 1.68	$ 8.38	$ 7.91	$ 13.36
Stock options excluded as their inclusion would be anti-dilutive									0.5	0.3	1.6