Fair Value Measurements (Fair value of financial instruments) (Details) - USD ($) $ in Millions		Jun. 30, 2015	Dec. 31, 2014
Level 1 | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior and senior subordinated notes		$ 5,991	$ 6,063
Level 1 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Senior and senior subordinated notes		6,090	6,390
Level 2 | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions
4 percent Convertible Senior Notes	[1]	7	32
Level 2 | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
4 percent Convertible Senior Notes	[1]	$ 8	$ 33
Convertible subordinated notes—4 percent
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Effective interest rate		6.90%
Debt instrument, convertible, if-converted value		$ 60
Convertible senior notes | Convertible subordinated notes—4 percent
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Stated interest rate		4.00%

[1] The fair value of the 4 percent Convertible Senior Notes is based on the market value of comparable notes. Consistent with the carrying amount, the fair value excludes the equity component of the notes. To exclude the equity component and calculate the fair value, we used an effective interest rate of 6.9 percent. As discussed below (see Item 3- Quantitative and Qualitative Disclosures about Market Risk), the total cost to settle the notes based on the closing price of our common stock on June 30, 2015 would be $60.