Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic EPS:
Net income	$ 185	$ 159
Weighted average common shares outstanding (in shares)	451,951	472,297
Basic EPS (in dollars per share)	$ 0.41	$ 0.34
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 185	$ 159
Share computation for diluted EPS:
Basic weighted average common shares outstanding (in shares)	451,951	472,297
Dilutive effect of stock awards (in shares)	1,478	2,301
Diluted weighted average common shares outstanding (in shares)	453,429	474,598
Diluted EPS (in dollars per share)	$ 0.41	$ 0.34
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in shares)	2,267	1