Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment charges	$ 11	$ 300	$ 0
Gain (loss) associated with Maine fossil	21	(67)	0
NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capacity of Spain Solar projects - in megawatts	99.8
Maine fossil [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capacity associated with assets previously held for sale (in megawatts)	796
Gain (loss) associated with Maine fossil	21	(67)
After-tax gain (loss) on assets previously held for sale	12	(43)
Fair Value, Measurements, Nonrecurring [Member] | NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of property, plant, and equipment prior to impairment charge	800
Property, plant, and equipment, fair value disclosure	500
Impairment charges		300
Impairment charges, after-tax		$ 342