Earnings Per Share (Details) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net income available to common stockholders	$ 115	$ 60
Denominator:
Denominator for basic earnings per share—weighted-average common shares (in shares)	97,007	95,225
Effect of dilutive securities:
Convertible subordinated notes—4 percent (in shares)	1,185	10,224
Denominator for diluted earnings per share—adjusted weighted-average common shares (in shares)	99,065	106,424
Basic earnings per share (in dollars per share)	$ 1.19us-gaap_EarningsPerShareBasic	$ 0.63us-gaap_EarningsPerShareBasic
Diluted earnings per share (in dollars per share)	$ 1.16us-gaap_EarningsPerShareDiluted	$ 0.56us-gaap_EarningsPerShareDiluted
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%
Employee stock options and warrants
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	336	435
Restricted stock units
Effect of dilutive securities:
Effect of dilutive securities, share-based payment arrangements (in shares)	537	540

Contractual interest rate for funds borrowed, under the debt agreement.

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