Earnings Per Common Share - Computation of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic EPS:
Net income	$ 624	$ 737	$ 1,661	$ 2,387
Weighted average common shares outstanding (in thousands)	484,772	525,415	493,164	564,886
Basic EPS (in dollars per share)	$ 1.29	$ 1.40	$ 3.37	$ 4.23
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 624	$ 737	$ 1,661	$ 2,387
Share computation for diluted EPS (in thousands):
Basic weighted average common shares outstanding	484,772	525,415	493,164	564,886
Dilutive effect of stock awards	1,853	3,095	2,632	3,793
Diluted weighted average common shares outstanding	486,625	528,510	495,796	568,679
Diluted EPS (in dollars per share)	$ 1.28	$ 1.40	$ 3.35	$ 4.20
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive	66	1,623	432	1,771