Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income available to common stockholders	$ 270	$ 270	$ 445	$ 453
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	78,264	83,456	78,829	83,854
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	78,467	84,199	79,240	84,723
Basic earnings per share (in dollars per share)	$ 3.45	$ 3.22	$ 5.65	$ 5.40
Diluted earnings per share (in dollars per share)	$ 3.44	$ 3.20	$ 5.62	$ 5.34
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	108	370	200	393
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	95	373	211	476