Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Antidilutive securities excluded (in shares)				600,000
Stated interest rate	4.00%		4.00%
Numerator:
Net income available to common stockholders	$ 94	$ 83	$ 154	$ 104
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	97,002,000	93,895,000	96,118,000	93,604,000
Effect of dilutive securities:
Employee stock options and warrants (in shares)	413,000	498,000	425,000	557,000
Convertible subordinated notes—4 percent (in shares)	7,758,000	11,946,000	9,005,000	11,908,000
Restricted stock units (in shares)	421,000	376,000	475,000	488,000
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	105,594,000	106,715,000	106,023,000	106,557,000
Basic earnings per share (in dollars per share)	$ 0.98	$ 0.89	$ 1.61	$ 1.12
Diluted earnings per share (in dollars per share)	$ 0.90	$ 0.78	$ 1.46	$ 0.98
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%