Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic EPS:
Net income	$ 341	$ 661	$ 1,094	$ 1,865
Weighted average common shares outstanding (in thousands) (in shares)	460,526	484,772	465,452	493,164
Basic EPS (in dollars per share)	$ 0.74	$ 1.36	$ 2.35	$ 3.78
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 341	$ 661	$ 1,094	$ 1,865
Share computation for diluted EPS (in thousands):
Basic weighted average common shares outstanding (in shares)	460,526	484,772	465,452	493,164
Dilutive effect of stock awards (in shares)	981	1,853	1,456	2,632
Diluted weighted average common shares outstanding (in shares)	461,507	486,625	466,908	495,796
Diluted EPS (in dollars per share)	$ 0.74	$ 1.36	$ 2.34	$ 3.76
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in thousands) (in shares)	1,694	66	1,128	432