Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income available to common stockholders	$ 208	$ 391	$ 593	$ 836
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	72,190	76,699	72,795	78,111
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	72,442	76,857	73,000	78,441
Basic earnings per share (in dollars per share)	$ 2.88	$ 5.10	$ 8.14	$ 10.70
Diluted earnings per share (in dollars per share)	$ 2.87	$ 5.08	$ 8.12	$ 10.66
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	9	30	12	144
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	243	128	193	186