Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income available to common stockholders									$ 890	$ 1,174	$ 1,096
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)									72,658	77,341	82,652
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)									72,929	77,710	83,530
Basic earnings per share (in dollars per share)	$ 4.11	$ 2.88	$ 2.94	$ 2.33	$ 4.51	$ 5.10	$ 3.45	$ 2.21	$ 12.24	$ 15.18	$ 13.26
Diluted earnings per share (in dollars per share)	$ 4.09	$ 2.87	$ 2.93	$ 2.33	$ 4.49	$ 5.08	$ 3.44	$ 2.19	$ 12.20	$ 15.11	$ 13.12
Employee stock option
Effect of dilutive securities:
Share-based payment arrangements (in shares)									12	114	379
Restricted stock units
Effect of dilutive securities:
Share-based payment arrangements (in shares)									259	255	499