Earnings Per Common Share - Computation of Basic and Diluted Earnings per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic EPS:
Net income	$ 186	$ 340
Weighted average common shares outstanding (in thousands) (in shares)	472,297	503,902
Basic EPS (in dollars per share)	$ 0.39	$ 0.67
Diluted EPS:
Net income for purposes of computing diluted EPS	$ 186	$ 340
Share computation for diluted EPS (in thousands):
Basic weighted average common shares outstanding (in shares)	472,297	503,902
Dilutive effect of stock awards (in shares)	2,301	3,895
Diluted weighted average common shares outstanding (in shares)	474,598	507,797
Diluted EPS (in dollars per share)	$ 0.39	$ 0.67
Restricted stock unit awards excluded from the calculation of diluted EPS because inclusion would be antidilutive (in thousands) (in shares)	1	395