Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income available to common stockholders									$ 1,346	$ 566	$ 585
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)									84,599	87,217	95,170
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)									0		660
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)									85,562	87,775	96,379
Basic earnings per share (in dollars per share)	$ 10.60	$ 2.36	$ 1.67	$ 1.29	$ 1.82	$ 2.18	$ 1.52	$ 1.01	$ 15.91	$ 6.49	$ 6.14
Diluted earnings per share (in dollars per share)	$ 10.45	$ 2.33	$ 1.65	$ 1.27	$ 1.80	$ 2.16	$ 1.52	$ 1.01	$ 15.73	$ 6.45	$ 6.07
Employee stock options and warrants
Effect of dilutive securities:
Share-based payment arrangements (in shares)									403	277	300
Restricted stock units
Effect of dilutive securities:
Share-based payment arrangements (in shares)									560	281	249
Convertible debt | 4 percent Convertible Senior Notes
Effect of dilutive securities:
Stated interest rate	4.00%				4.00%				4.00%	4.00%	4.00%