Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income available to common stockholders									$ 1,096	$ 1,346	$ 566
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)									82,652	84,599	87,217
Effect of dilutive securities:
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)									83,530	85,562	87,775
Basic earnings per share (in dollars per share)	$ 3.84	$ 4.05	$ 3.22	$ 2.18	$ 10.60	$ 2.36	$ 1.67	$ 1.29	$ 13.26	$ 15.91	$ 6.49
Diluted earnings per share (in dollars per share)	$ 3.80	$ 4.01	$ 3.20	$ 2.15	$ 10.45	$ 2.33	$ 1.65	$ 1.27	$ 13.12	$ 15.73	$ 6.45
Employee stock options and warrants
Effect of dilutive securities:
Share-based payment arrangements (in shares)									379	403	277
Restricted stock units
Effect of dilutive securities:
Share-based payment arrangements (in shares)									499	560	281