Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS:
Net income	$ 662	$ 185	$ 556	$ 159	$ 847	$ 716
Weighted average common shares outstanding (in shares)	445,008		463,533		448,479	467,915
Basic EPS (in dollars per share)	$ 1.49		$ 1.20		$ 1.89	$ 1.53
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 662		$ 556		$ 847	$ 716
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	445,008		463,533		448,479	467,915
Dilutive effect of stock awards (in shares)	579		1,085		1,029	1,693
Diluted weighted average common shares outstanding (in shares)	445,587		464,618		449,508	469,608
Diluted EPS (in dollars per share)	$ 1.49		$ 1.20		$ 1.88	$ 1.52
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)	3,087		1,690		2,677	845