Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income available to common stockholders	$ 183	$ 109
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	84,256	84,456
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	85,238	85,377
Basic earnings per share (in dollars per share)	$ 2.18	$ 1.29
Diluted earnings per share (in dollars per share)	$ 2.15	$ 1.27
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	417	414
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	565	507