Accounting for Derivative Instruments and Hedging Activities (Details 2 - Offsetting Derivatives) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative Assets	$ 2,578	$ 2,220
Cash Collateral (Held) / Posted	(100)	(106)
Gross Amounts of Recognized Derivative Liabilities	(2,574)	(2,214)
Cash Collateral Posted	411	568
Gross Amounts of Recognized Assets / Liabilities	4	6
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	87	158
Derivative Asset, Fair Value, Amount Offset Against Collateral Net Of Derivative Liability, Fair Value, Amount Offset Against Collateral	91	164
Commodity contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative Assets	2,578	2,220
Derivative Instruments	(2,000)	(1,616)
Cash Collateral (Held) / Posted	(99)	(113)
Net Amount	479	491
Gross Amounts of Recognized Derivative Liabilities	(2,389)	(2,086)
Derivative Instruments	2,000	1,616
Cash Collateral Posted	186	271
Derivative Liability, Fair Value, Amount Offset Against Collateral	(203)	(199)
Gross Amounts of Recognized Assets / Liabilities	189	134
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	87	158
Derivative Asset, Fair Value, Amount Offset Against Collateral Net Of Derivative Liability, Fair Value, Amount Offset Against Collateral	276	292
Interest rate contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gross Amounts of Recognized Derivative Liabilities	(185)	(128)
Derivative Instruments	0	0
Cash Collateral Posted	0	0
Derivative Liability, Fair Value, Amount Offset Against Collateral	$ (185)	$ (128)