Effect of Derivative Contracts on the Income Statement (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest rate swap agreements \| Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	$ 0	$ 63	$ (15)
Energy commodity derivative contracts(a)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	4	30	176
Derivative, Gain (Loss) on Derivative	57	73	31
Energy commodity derivative contracts(a) \| Revenues—Natural gas sales
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	20	(10)	17
Energy commodity derivative contracts(a) \| Revenues—Product sales and other
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	(16)	(26)	176
Energy commodity derivative contracts(a) \| Costs of sales
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0	3	(2)
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Loss to be reclassified within twelve months	1
Designated as Hedging Instrument [Member] \| Other Comprehensive Income (Loss) [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	145	(104)	164
Designated as Hedging Instrument [Member] \| Operating Income (Loss) [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	171	116	272
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	11	(12)	2
Designated as Hedging Instrument [Member] \| Interest rate swap agreements \| Other Comprehensive Income (Loss) [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	0	(2)	(4)
Designated as Hedging Instrument [Member] \| Interest rate swap agreements \| Interest Expense [Member] \| Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(103)	(180)	25
Designated as Hedging Instrument [Member] \| Interest rate swap agreements \| Interest Expense [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	(3)	(3)	(3)
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0
Designated as Hedging Instrument [Member] \| Interest Rate Contract [Member] \| Interest Expense [Member] \| Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	105	160	(33)
Designated as Hedging Instrument [Member] \| Energy commodity derivative contracts(a) \| Other Comprehensive Income (Loss) [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	24	(115)	201
Designated as Hedging Instrument [Member] \| Energy commodity derivative contracts(a) \| Revenues—Natural gas sales \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	12	15	54
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0
Designated as Hedging Instrument [Member] \| Energy commodity derivative contracts(a) \| Revenues—Product sales and other \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	35	148	236
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	11	(12)	2
Designated as Hedging Instrument [Member] \| Energy commodity derivative contracts(a) \| Costs of sales \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	9	(17)	(15)
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0
Designated as Hedging Instrument [Member] \| Currency Swap [Member] \| Other Comprehensive Income (Loss) [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net	121	13	(33)
Designated as Hedging Instrument [Member] \| Other Credit Derivatives [Member] \| Other Expense [Member] \| Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain/(loss)reclassified fromAccumulated OCIinto income(effective portion)	118	(27)	0
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	$ 0	$ 0	$ 0

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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