Accounting for Derivative Instruments and Hedging Activities (Details) bbl in Millions, T in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MWh MMBTU T bbl	Dec. 31, 2015 USD ($) MWh MMBTU T bbl
Fair value of the derivative instrument
Derivative assets	$ 1,251	$ 2,220
Derivative liabilities	1,378	2,214
Derivatives Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	12	0
Derivative liabilities	69	110
Derivatives Not Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	1,239	2,220
Derivative liabilities	1,309	2,104
Interest rate contracts current | Derivatives Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	28	42
Interest rate contracts current | Derivatives Not Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	7	5
Interest rate contracts long-term | Derivatives Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	12	0
Derivative liabilities	41	68
Interest rate contracts long-term | Derivatives Not Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	37	0
Derivative liabilities	12	13
Commodity contracts current | Derivatives Not Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	1,062	1,915
Derivative liabilities	1,049	1,674
Commodity contracts long-term | Derivatives Not Designated as Cash Flow Hedges
Fair value of the derivative instrument
Derivative assets	140	305
Derivative liabilities	$ 241	$ 412
Emissions [Member] | Short Ton
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Mass | T	0	1
Equity [Member] | Shares [Member]
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Notional Amount	$ 1	$ 1
Interest [Member] | Dollars
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Notional Amount	$ 3,429	$ 2,326
Coal [Member] | Short Ton
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Mass | T	41	35
Natural Gas [Member] | MMbtu
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	85	293
Oil [Member] | Barrel
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Volume | bbl	1	1
Power [Member] | MWh
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	(28)	(74)
Short [Member] | MW/Day [Member] | Capacity [Member]
Volumetric Underlying Derivative Transactions [Abstract]
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh	(1)	(1)