Accounting for Derivative Instruments and Hedging Activities (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net	$ 0	$ 0	$ 0
Derivative assets	1,251	2,220
Cash Collateral (Held)	(2)	(106)
Gross Amounts of Recognized Derivative Liabilities	(1,378)	(2,214)
Cash collateral posted in support of energy risk management activities	203	568
Gross Amounts of Recognized Assets / Liabilities	(127)	6
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	13	158
Net Amount	(114)	164
Commodity contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	1,202	2,220
Derivative Instruments	(1,005)	(1,616)
Cash Collateral (Held)	(1)	(113)
Net Amount	196	491
Gross Amounts of Recognized Derivative Liabilities	(1,290)	(2,086)
Derivative Instruments	1,005	1,616
Cash collateral posted in support of energy risk management activities	14	271
Net Amount	(271)	(199)
Gross Amounts of Recognized Assets / Liabilities	(88)	134
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	13	158
Net Amount	(75)	292
Interest rate contracts
Offsetting of Derivatives by Counterparty Master Agreement Level and Collateral Received or Paid
Derivative assets	49
Derivative Instruments	(4)
Cash Collateral (Held)	0
Net Amount	45
Gross Amounts of Recognized Derivative Liabilities	(88)	(128)
Derivative Instruments	4	0
Cash collateral posted in support of energy risk management activities	0	0
Net Amount	(84)	$ (128)
Gross Amounts of Recognized Assets / Liabilities	(39)
Derivative Instruments	0
Cash Collateral (Held) / Posted	0
Net Amount	$ (39)