Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net income available to common stockholders	$ 187	$ 215	$ 413	$ 416
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	85,945	94,213	88,175	95,992
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)	0	574	0	786
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	86,445	95,191	88,624	97,285
Basic earnings per share (in dollars per share)	$ 2.18	$ 2.28	$ 4.68	$ 4.33
Diluted earnings per share (in dollars per share)	$ 2.16	$ 2.25	$ 4.66	$ 4.27
Convertible senior notes | Convertible subordinated notes—4 percent
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Stated interest rate	4.00%	4.00%	4.00%	4.00%
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	278	291	281	311
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	222	113	168	196