Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator:
Net income available to common stockholders	$ 134	$ 86	$ 226	$ 201
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	88,095	96,647	89,303	96,896
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)	0	605	0	894
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	88,473	97,747	89,709	98,416
Basic earnings per share (in dollars per share)	$ 1.52	$ 0.89	$ 2.53	$ 2.07
Diluted earnings per share (in dollars per share)	$ 1.52	$ 0.88	$ 2.52	$ 2.04
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)	271	298	267	318
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	107	197	139	308