Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income available to common stockholders	$ 212	$ 270	$ 385	$ 445
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	72,161	78,264	73,101	78,829
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	72,275	78,467	73,270	79,240
Basic earnings per share (in dollars per share)	$ 2.94	$ 3.45	$ 5.26	$ 5.65
Diluted earnings per share (in dollars per share)	$ 2.93	$ 3.44	$ 5.25	$ 5.62
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	12	108	14	200
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	102	95	155	211