Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount											300
Numerator:
Net income available to common stockholders									$ 585	$ 540	$ 387
Denominator:
Denominator for basic earnings per share—weighted-average common shares									95,170	97,489	93,436
Effect of dilutive securities:
4 percent Convertible Senior Notes									660	6,386	11,769
Denominator for diluted earnings per share—adjusted weighted-average common shares									96,379	104,956	106,291
Basic earnings per share (in dollars per share)	$ 1.82	$ 2.28	$ 0.89	$ 1.19	$ 1.96	$ 1.95	$ 0.98	$ 0.63	$ 6.14	$ 5.54	$ 4.14
Diluted earnings per share (in dollars per share)	$ 1.81	$ 2.25	$ 0.88	$ 1.16	$ 1.88	$ 1.84	$ 0.90	$ 0.56	$ 6.07	$ 5.15	$ 3.64
4 percent Convertible Senior Notes
Effect of dilutive securities:
Stated interest rate	4.00%				4.00%				4.00%	4.00%	4.00%
Employee stock options and warrants
Effect of dilutive securities:
Share-based payment arrangements									300	394	504
Restricted stock units
Effect of dilutive securities:
Share-based payment arrangements									249	687	582