Debt - Senior Notes, Contingent Convertible Senior Notes and Convertible Senior Notes Narrative (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) shares	Dec. 31, 2016 USD ($) $ / €	Dec. 31, 2006 $ / €
Long-Term Debt Instrument [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent	100.00%		100.00%
Debt Instrument, Face Amount	$ 9,710		$ 9,710		$ 9,989
Debt Instrument, Repurchased Face Amount	1,604	$ 558	1,604	$ 558
Cash paid to purchase debt			1,746	472
Long-term Debt, Gross	9,853		9,853		10,441
Gains on purchases or exchanges of debt	$ 191	68	$ 184	168
Convertible Debt [Member] | Privately Negotiated Exchanges for Common Stock [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount		$ 577		$ 577
Convertible Debt [Member] | Common Stock [Member]
Long-Term Debt Instrument [Line Items]
Debt Conversion, Converted Instrument, Shares Issued | shares				109,351,707
Convertible Debt [Member] | 2.5% Contingent Convertible Senior Notes Due 2037 [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.50%		2.50%
Debt Instrument, Interest Rate, Effective Percentage	8.00%		8.00%
Debt Instrument, Face Amount	$ 1		$ 1		114
Long-term Debt, Gross	$ 1		$ 1		112
Convertible Debt [Member] | 2.25% Contingent Convertible Senior Notes Due 2038 [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.25%		2.25%
Debt Instrument, Interest Rate, Effective Percentage	8.00%		8.00%
Debt Instrument, Face Amount	$ 9		$ 9		200
Long-term Debt, Gross	$ 8		$ 8		180
Convertible Debt [Member] | 5.5% Convertible Senior Notes Due 2026 [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.50%		5.50%
Debt Instrument, Interest Rate, Effective Percentage	11.50%		11.50%
Debt Instrument, Face Amount	$ 1,250		$ 1,250		1,250
Long-term Debt, Gross	$ 824		$ 824		811
Senior Notes [Member] | 8.00% Senior Notes due 2027 [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	8.00%		8.00%
Debt Instrument, Face Amount	$ 750		$ 750		0
Proceeds from Issuance of Senior Long-term Debt	742
Debt Instrument, Redemption, Description			Some or all of the notes may be redeemed at any time prior to June 15, 2022, subject to a make-whole premium. We also may redeem some or all of the notes at any time on or after June 15, 2022, at the applicable redemption price in accordance with the terms of the notes and the indenture and supplemental indenture governing the notes.
Debt Instrument, Redemption Price, Percentage of Principal Amount Redeemed			35.00%
Debt Instrument, Redemption Period, Start Date			Jun. 15, 2020
Debt Instrument, Redemption Price, Percentage			108.00%
Long-term Debt, Gross	$ 750		$ 750		0
Senior Notes [Member] | 6.25% Euro-Denominated Senior Notes Due 2017 [ Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.25%		6.25%
Debt Instrument, Face Amount	$ 0		$ 0		258
Long-term Debt, Gross	$ 0		$ 0		$ 258
Senior Notes [Member] | 6.25% Euro-Denominated Senior Notes Due 2017 [ Member] | Cross Currency Interest Rate Contract [Member]
Long-Term Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.25%		6.25%		6.25%
Derivative, Forward Exchange Rate | $ / €					1.0517	1.3325