Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income available to common stockholders	$ 391	$ 333	$ 836	$ 786
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	76,699	82,344	78,111	83,345
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	76,857	83,172	78,441	84,211
Basic earnings per share (in dollars per share)	$ 5.10	$ 4.05	$ 10.70	$ 9.44
Diluted earnings per share (in dollars per share)	$ 5.08	$ 4.01	$ 10.66	$ 9.34
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	30	372	144	389
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	128	456	186	477