Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income available to common stockholders	$ 175	$ 183
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	79,401	84,256
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	80,047	85,238
Basic earnings per share (in dollars per share)	$ 2.21	$ 2.18
Diluted earnings per share (in dollars per share)	$ 2.19	$ 2.15
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	294	417
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	352	565