Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Numerator:
Net income available to common stockholders	$ 92	$ 115
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	90,510	97,007
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)	0	1,185
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	90,943	99,065
Basic earnings per share (in dollars per share)	$ 1.01	$ 1.19
Diluted earnings per share (in dollars per share)	$ 1.01	$ 1.16
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%
Employee stock options
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	263	336
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	170	537