Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic EPS:
Net income	$ 566	$ 864	$ 753	$ 1,204
Weighted average common shares outstanding (in thousands) (in shares)	463,533	490,818	467,915	497,360
Basic EPS (in dollars per share)	$ 1.22	$ 1.76	$ 1.61	$ 2.42
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 566	$ 864	$ 753	$ 1,204
Share computation for diluted EPS (in thousands):
Basic weighted average common shares outstanding (in shares)	463,533	490,818	467,915	497,360
Dilutive effect of stock awards (in shares)	1,085	2,147	1,693	3,021
Diluted weighted average common shares outstanding (in shares)	464,618	492,965	469,608	500,381
Diluted EPS (in dollars per share)	$ 1.22	$ 1.75	$ 1.60	$ 2.41
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in thousands) (in shares)	1,690	837	845	616