Accounting for Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Derivative
Net notional amount buy/(sell) of open derivative transactions	$ 2,251	$ 2,121
Fair value of the derivative instrument
Derivative assets	2,986	4,666
Derivative liabilities	3,023	4,215
Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	1	318
Derivative liabilities	110	108
Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	2,985	4,348
Derivative liabilities	2,913	4,107
Interest rate contracts current | Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	11	39
Interest rate contracts current | Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	13	0
Interest rate contracts long-term | Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	96	68
Interest rate contracts long-term | Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	14	1
Commodity contracts current | Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	1	318
Derivative liabilities	2	0
Commodity contracts current | Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	2,676	3,898
Derivative liabilities	2,436	3,712
Commodity contracts long-term | Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	0	0
Derivative liabilities	1	1
Commodity contracts long-term | Not Designated as Hedging Instrument [Member]
Fair value of the derivative instrument
Derivative assets	309	450
Derivative liabilities	$ 450	$ 394
Emissions
Derivative
Net notional mass buy/(sell) of open derivative transactions	(1,000,000)	(2,000,000)
Coal
Derivative
Net notional mass buy/(sell) of open derivative transactions	34,000,000	37,000,000
Natural Gas
Derivative
Net notional energy buy/(sell) of open derivative transactions	(244,000,000)	13,000,000
Oil
Derivative
Net notional volume buy/(sell) of open derivative transactions	0	1,000,000
Power
Derivative
Net notional energy buy/(sell) of open derivative transactions	12,000,000	4,000,000