Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded (in shares)				400,000
Numerator:
Net income available to common stockholders	$ 192	$ 143	$ 346	$ 247
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	98,485,000	93,244,000	96,916,000	93,483,000
Effect of dilutive securities:
Employee stock options and warrants (in shares)	376,000	463,000	407,000	527,000
Convertible subordinated notes—4 percent (in shares)	4,748,000	11,407,000	7,606,000	11,744,000
Restricted stock units (in shares)	467,000	434,000	465,000	516,000
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	104,076,000	105,548,000	105,394,000	106,270,000
Basic earnings per share (in dollars per share)	$ 1.95	$ 1.53	$ 3.57	$ 2.65
Diluted earnings per share (in dollars per share)	$ 1.84	$ 1.35	$ 3.29	$ 2.33
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%