Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income available to common stockholders	$ 270	$ 141	$ 453	$ 250
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	83,456	84,635	83,854	84,546
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	84,199	85,408	84,723	85,401
Basic earnings per share (in dollars per share)	$ 3.22	$ 1.67	$ 5.40	$ 2.95
Diluted earnings per share (in dollars per share)	$ 3.20	$ 1.65	$ 5.34	$ 2.92
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	370	394	393	403
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	373	379	476	452