Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator:
Net income available to common stockholders	$ 141	$ 134	$ 250	$ 226
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	84,635	88,095	84,546	89,303
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	85,408	88,473	85,401	89,709
Basic earnings per share (in dollars per share)	$ 1.67	$ 1.52	$ 2.95	$ 2.53
Diluted earnings per share (in dollars per share)	$ 1.65	$ 1.52	$ 2.92	$ 2.52
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	394	271	403	267
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	379	107	452	139