Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator:
Net income available to common stockholders	$ 215	$ 192	$ 416	$ 346
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	94,213	98,485	95,992	96,916
Effect of dilutive securities:
4 percent Convertible Senior Notes (in shares)	574	4,748	786	7,606
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	95,191	104,076	97,285	105,394
Basic earnings per share (in dollars per share)	$ 2.28	$ 1.95	$ 4.33	$ 3.57
Diluted earnings per share (in dollars per share)	$ 2.25	$ 1.84	$ 4.27	$ 3.29
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)	291	376	311	407
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	113	467	196	465