Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income available to common stockholders									$ 566	$ 585	$ 540
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)									87,217	95,170	97,489
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)									0	660	6,386
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)									87,775	96,379	104,956
Basic earnings per share (in dollars per share)	$ 1.82	$ 2.18	$ 1.52	$ 1.01	$ 1.82	$ 2.28	$ 0.89	$ 1.19	$ 6.49	$ 6.14	$ 5.54
Diluted earnings per share (in dollars per share)	$ 1.80	$ 2.16	$ 1.52	$ 1.01	$ 1.81	$ 2.25	$ 0.88	$ 1.16	$ 6.45	$ 6.07	$ 5.15
4 percent Convertible Senior Notes
Effect of dilutive securities:
Stated interest rate					4.00%					4.00%	4.00%
Employee stock options and warrants
Effect of dilutive securities:
Share-based payment arrangements (in shares)									277	300	394
Restricted stock units
Effect of dilutive securities:
Share-based payment arrangements (in shares)									281	249	687