Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Finite Lived Intangible Assets [Line Items]
Effective tax rate from continuing operations	14.00%	34.00%	17.00%	34.00%
Reduction of income tax expense	$ (24)	$ 44	$ 79	$ 160
Significant Accounting Policies (Textuals) [Abstract]
Intangible assets related to emission allowances	9		9		9
Renewable Energy Credits	23		23		17
Gross Carrying amount of accumulated amortization of intangible assets	64		64		63
Accumulated amortization of intangible assets	25		25		22
Unrecognized Tax Benefits	3		3		5
Total cash collateral posted, net of cash collateral received	68		68
Derivative assets, net of collateral	6		6
Derivative liabilities, net of collateral	34		34
Cash collateral received not related to unrealized derivative positions included in account receivable	4		4
Cash collateral paid not related to unrealized derivative positions included in account payable	44		44
Increase in unrecognized tax benefits			40
Decrease in unrecognized tax benefits	49		49
Reduction of net deferred tax assets	307		307
Decrease in deferred tax liabilities attributable to regulated utilities	41		41
Decrease in deferred tax assets attributable to regulated utilities	395		395
Amount due to a lower apportionment factor	88		88
Contract intangible assets [Member]
Finite Lived Intangible Assets [Line Items]
Finite Lived Intangible Assets Useful Life Minimum			4
Finite Lived Intangible Assets Useful Life Maximum			30
MCIT [Member]
Finite Lived Intangible Assets [Line Items]
Effective tax rate from continuing operations			6.00%
Reduction of income tax expense			$ 88