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Accounting for Derivative Instruments and Hedging Activities - Offsetting of derivatives by counterparty master agreement level and collateral received (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Cash Collateral (Held) / Posted	$ (30)	$ (37)
Cash Collateral Posted	209	171
Gross Amounts of Recognized Assets / Liabilities	168	103
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	12	61
Net Amount	180	164
Commodity contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	1,022	744
Derivative Instruments	(765)	(578)
Cash Collateral (Held) / Posted	(18)	(11)
Net Amount	239	155
Derivative liabilities	(905)	(674)
Derivative Instruments	765	578
Cash Collateral Posted	30	72
Net Amount	(110)	(24)
Gross Amounts of Recognized Assets / Liabilities	117	70
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	12	61
Net Amount	129	131
Interest rate contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	53	39
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	0	0
Net Amount	53	39
Derivative liabilities	(2)	(6)
Derivative Instruments	0	0
Cash Collateral Posted	0	0
Net Amount	(2)	(6)
Gross Amounts of Recognized Assets / Liabilities	51	33
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	0	0
Net Amount	$ 51	$ 33

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