Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator:
Net income available to common stockholders	$ 109	$ 92
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	84,456	90,510
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	85,377	90,943
Basic earnings per share (in dollars per share)	$ 1.29	$ 1.01
Diluted earnings per share (in dollars per share)	$ 1.27	$ 1.01
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	414	263
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	507	170