Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of hydropower generation plants through assumption of debt by buyer			$ 0	$ 700
GAIN FROM DISCONTINUED OPERATIONS, NET OF INCOME TAXES	0	0	0	231	[1],[2],[3]
Maine Hydro [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capacity associated with discontinued operations (in megawatts)		351		351
Sale of hydropower generation plants through assumption of debt by buyer				700
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax				372
GAIN FROM DISCONTINUED OPERATIONS, NET OF INCOME TAXES				$ 231

[1] Certain amounts were restated to conform to current year's presentation. See Note 4 - Nonrecurring Fair Value Measurements.

[2] Prior year amounts were restated to conform to current year's presentation. See Note 4 - Nonrecurring Fair Value Measurements

[3] See Note 6.