Accounting for Derivative Instruments and Hedging Activities (Details 2) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	$ 2,220	$ 2,905
Cash Collateral (Held)	(106)	(72)
Gross Amounts of Recognized Derivative Liabilities	(2,214)	(2,492)
Cash collateral paid in support of energy risk management activities	568	187
Gross Amounts of Recognized Assets / Liabilities	6	413
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	158	(45)
Net Amount	164	368
Commodity contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	2,220	2,902
Derivative Instruments	(1,616)	(2,155)
Cash Collateral (Held)	(113)	(72)
Net Amount	491	675
Gross Amounts of Recognized Derivative Liabilities	(2,086)	(2,327)
Derivative Instruments	1,616	2,155
Cash collateral paid in support of energy risk management activities	271	27
Net Amount	(199)	(145)
Gross Amounts of Recognized Assets / Liabilities	134	575
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	158	(45)
Net Amount	292	530
Interest rate contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets		2
Derivative Instruments		(2)
Cash Collateral (Held)		0
Net Amount		0
Gross Amounts of Recognized Derivative Liabilities	(128)	(165)
Derivative Instruments	0	2
Cash collateral paid in support of energy risk management activities	0	0
Net Amount	$ (128)	(163)
Gross Amounts of Recognized Assets / Liabilities		(163)
Derivative Instruments		0
Cash Collateral (Held) / Posted		0
Net Amount		(163)
Equity Contract [Member]
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets		1
Derivative Instruments		0
Cash Collateral (Held)		0
Net Amount		$ 1