Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income available to common stockholders	$ 86	$ 94	$ 201	$ 154
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	96,647	97,002	96,896	96,118
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)	605	7,758	894	9,005
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	97,747	105,594	98,416	106,023
Basic earnings per share (in dollars per share)	$ 0.89	$ 0.98	$ 2.07	$ 1.61
Diluted earnings per share (in dollars per share)	$ 0.88	$ 0.90	$ 2.04	$ 1.46
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 and warrants
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	298	413	318	425
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	197	421	308	475