Accounting for Derivative Instruments and Hedging Activities - Offsetting of derivatives by counterparty master agreement level and collateral received (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	$ 870	$ 1,248
Cash Collateral (Held) / Posted	(19)	(81)
Gross Amounts of Recognized Derivative Liabilities	(1,004)	(1,376)
Cash Collateral Posted	277	150
Gross Amounts of Recognized Assets / Liabilities	(134)	(128)
Derivative Asset Fair Value Gross Liability Net Of Derivative Liability Fair Value Gross Asset	0	0
Derivative, Collateral, Obligation to Return Cash Net Of Derivative, Collateral, Right to Reclaim Cash	119	0
Derivative Asset, Fair Value, Amount Offset Against Collateral Net Of Derivative Liability, Fair Value, Amount Offset Against Collateral	(15)	(128)
Commodity contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	831	1,199
Derivative Instruments	(730)	(1,021)
Cash Collateral (Held) / Posted	(2)	(13)
Net Amount	99	165
Gross Amounts of Recognized Derivative Liabilities	(909)	(1,288)
Derivative Instruments	730	1,021
Cash Collateral Posted	121	13
Derivative Liability, Fair Value, Amount Offset Against Collateral	(58)	(254)
Gross Amounts of Recognized Assets / Liabilities	(78)	(89)
Derivative Asset Fair Value Gross Liability Net Of Derivative Liability Fair Value Gross Asset	0	0
Derivative, Collateral, Obligation to Return Cash Net Of Derivative, Collateral, Right to Reclaim Cash	119	0
Derivative Asset, Fair Value, Amount Offset Against Collateral Net Of Derivative Liability, Fair Value, Amount Offset Against Collateral	41	(89)
Interest rate contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	39	49
Derivative Instruments	(2)	(4)
Cash Collateral (Held) / Posted	0	0
Net Amount	37	45
Gross Amounts of Recognized Derivative Liabilities	(95)	(88)
Derivative Instruments	2	4
Cash Collateral Posted	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	(93)	(84)
Gross Amounts of Recognized Assets / Liabilities	(56)	(39)
Derivative Asset Fair Value Gross Liability Net Of Derivative Liability Fair Value Gross Asset	0	0
Derivative, Collateral, Obligation to Return Cash Net Of Derivative, Collateral, Right to Reclaim Cash	0	0
Derivative Asset, Fair Value, Amount Offset Against Collateral Net Of Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (56)	$ (39)