Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income available to common stockholders	$ 199	$ 187	$ 449	$ 413
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	84,663	85,945	84,585	88,175
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	85,592	86,445	85,474	88,624
Basic earnings per share (in dollars per share)	$ 2.36	$ 2.18	$ 5.31	$ 4.68
Diluted earnings per share (in dollars per share)	$ 2.33	$ 2.16	$ 5.26	$ 4.66
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	398	278	401	281
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	531	222	488	168