Debt - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Long-Term Debt Instrument [Line Items]
Cash paid to purchase debt	$ 0		$ 982
Gains (losses) on restructuring of debt	0	$ 7	$ (7)
Debt instrument, face amount	9,400	9,981
Senior Notes, Sr. Secured Notes, Contingent Convertible Notes [Member]
Long-Term Debt Instrument [Line Items]
Debt instrument, repurchased face amount		908
Cash paid to purchase debt		982
Revolving credit facility [Member]
Long-Term Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity	3,800
Letters of credit outstanding, amount	157
Revolving credit facility [Member] | Line of Credit [Member]
Long-Term Debt Instrument [Line Items]
Debt instrument, face amount	$ 200	$ 781