Accounting for Derivative Instruments and Hedging Activities - Fair value within the derivative instrument valuation (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative Assets	$ 870	$ 1,248
Derivative Liabilities	1,004	1,376
Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	9	12
Derivative Liabilities	28	69
Designated as Hedging Instrument | Interest rate contracts current
Derivative [Line Items]
Derivative Assets	0	0
Derivative Liabilities	9	28
Designated as Hedging Instrument | Interest rate contracts long-term
Derivative [Line Items]
Derivative Assets	9	12
Derivative Liabilities	19	41
Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative Assets	861	1,236
Derivative Liabilities	976	1,307
Not Designated as Hedging Instrument | Interest rate contracts current
Derivative [Line Items]
Derivative Assets	4	0
Derivative Liabilities	21	7
Not Designated as Hedging Instrument | Interest rate contracts long-term
Derivative [Line Items]
Derivative Assets	26	37
Derivative Liabilities	46	12
Not Designated as Hedging Instrument | Commodity contracts current
Derivative [Line Items]
Derivative Assets	640	1,067
Derivative Liabilities	681	1,057
Not Designated as Hedging Instrument | Commodity contracts long-term
Derivative [Line Items]
Derivative Assets	191	132
Derivative Liabilities	$ 228	$ 231