Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic EPS:
Net income	$ 425	$ 662	$ 185	$ 372	$ 556	$ 159	$ 1,272	$ 1,088
Weighted average common shares outstanding (in shares)	441,915			460,526			446,291	465,452
Basic EPS (in dollars per share)	$ 0.96			$ 0.81			$ 2.85	$ 2.34
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 425			$ 372			$ 1,272	$ 1,088
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	441,915			460,526			446,291	465,452
Dilutive effect of stock awards (in shares)	486			981			848	1,456
Diluted weighted average common shares outstanding (in shares)	442,401			461,507			447,139	466,908
Diluted EPS (in dollars per share)	$ 0.96			$ 0.81			$ 2.84	$ 2.33
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)	2,590			1,694			2,648	1,128