Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income available to common stockholders	$ 333	$ 199	$ 786	$ 449
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	82,344	84,663	83,345	84,585
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	83,172	85,592	84,211	85,474
Basic earnings per share (in dollars per share)	$ 4.05	$ 2.36	$ 9.44	$ 5.31
Diluted earnings per share (in dollars per share)	$ 4.01	$ 2.33	$ 9.34	$ 5.26
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities (in shares)	372	398	389	401
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities (in shares)	456	531	477	488