Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic EPS:
Net income	$ 414	$ 425	$ 662	$ 185	$ 325	$ 372	$ 556	$ 159	$ 1,686	$ 1,412	$ 1,282
Weighted average common shares outstanding (in shares)	434,578	441,915	445,008	451,951	460,589	460,526	463,533	472,297	443,363	464,236	489,164
Basic EPS (in dollars per share)	$ 0.95	$ 0.96	$ 1.49	$ 0.41	$ 0.71	$ 0.81	$ 1.20	$ 0.34	$ 3.80	$ 3.04	$ 2.62
Diluted EPS:
Net income for purposes of computing diluted EPS									$ 1,686	$ 1,412	$ 1,282
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	434,578	441,915	445,008	451,951	460,589	460,526	463,533	472,297	443,363	464,236	489,164
Dilutive effect of stock awards (in shares)									906	1,424	2,528
Diluted weighted average common shares outstanding (in shares)	435,659	442,401	445,587	453,429	461,915	461,507	464,618	474,598	444,269	465,660	491,692
Diluted EPS (in dollars per share)	$ 0.95	$ 0.96	$ 1.49	$ 0.41	$ 0.70	$ 0.81	$ 1.20	$ 0.34	$ 3.79	$ 3.03	$ 2.61
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)									2,520	1,266	328