Debt Debt - Conversion Value in Excess of Principal (Details) - Jun. 04, 2015 $ / shares in Units, shares in Millions, $ in Millions		USD ($) d $ / shares shares
Schedule of Conversion Value in Excess of Principal [Line Items]
Share Price (in dollars per share) | $ / shares		$ 26.99
Convertible Debt [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Long-term Debt, Gross		$ 2,019
Debt Instrument Convertible Underlying Shares | shares		130
Debt Instrument, Convertible, If-converted Value in Excess of Principal		$ 1,576
Debt Instrument, Convertible, Threshold Trading Days | d		20
Debt Instrument, Convertible, Threshold Consecutive Trading Days		30 days
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger		130.00%
Convertible Debt [Member] | 2032C convertible senior notes [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Debt Instrument Put Date Earliest	[1]	May 31, 2019
Long-term Debt, Gross		$ 224
Debt Instrument Convertible Underlying Shares | shares		23
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 9.63
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	[2]	$ 12.52
Debt Instrument, Convertible, If-converted Value in Excess of Principal	[3]	$ 403
Convertible Debt [Member] | 2032D convertible senior notes [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Debt Instrument Put Date Earliest	[1]	May 31, 2021
Long-term Debt, Gross		$ 177
Debt Instrument Convertible Underlying Shares | shares		18
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 9.98
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	[2]	$ 12.97
Debt Instrument, Convertible, If-converted Value in Excess of Principal	[3]	$ 302
Convertible Debt [Member] | 2033E convertible senior notes [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Debt Instrument Put Date Earliest	[1]	Feb. 28, 2018
Long-term Debt, Gross		$ 293
Debt Instrument Convertible Underlying Shares | shares		27
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 10.93
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	[2]	$ 14.21
Debt Instrument, Convertible, If-converted Value in Excess of Principal	[3]	$ 430
Convertible Debt [Member] | 2033F convertible senior note [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Debt Instrument Put Date Earliest	[1]	Feb. 28, 2020
Long-term Debt, Gross		$ 300
Debt Instrument Convertible Underlying Shares | shares		27
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 10.93
Debt Instrument, Convertible, Stock Price Trigger | $ / shares	[2]	$ 14.21
Debt Instrument, Convertible, If-converted Value in Excess of Principal	[3]	$ 441
Convertible Debt [Member] | 2043G convertible senior notes [Member]
Schedule of Conversion Value in Excess of Principal [Line Items]
Debt Instrument Put Date Earliest	[1],[4]	Nov. 30, 2028
Long-term Debt, Gross	[4]	$ 1,025
Debt Instrument Convertible Underlying Shares | shares		35
Debt Instrument, Convertible, Conversion Price | $ / shares		$ 29.16
Debt Instrument, Convertible, Stock Price Trigger | $ / shares		$ 37.91
Debt Instrument, Convertible, If-converted Value in Excess of Principal	[3]	$ 0
Debt Instrument Issued At A Discount, Original Principal Amount		820
Debt Instrument, Scheduled Accreted Principal Amount		$ 917

[1] The terms of our convertible notes give holders the right to require us to repurchase all or a portion of their notes at a date prior to the contractual maturities of the notes.

[2] Holders have the right to convert all or a portion of their notes at a date prior to the contractual maturity if, during any calendar quarter, the closing price of our common stock for at least 20 trading days in the 30 consecutive trading days ending on the last trading day of the preceding calendar quarter is more than 130% of the conversion price. The closing price of our common stock exceeded the thresholds for the calendar quarter ended June 30, 2015 for our 2032 Notes and 2033 Notes; therefore, those notes are convertible by the holders through September 30, 2015.

[3] Based on our closing share price of $26.99 as of June 4, 2015.

[4] The original principal amount of $820 million accretes up to $917 million at the holder put date in November 2028 and $1.03 billion at maturity in 2043.