Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) MW	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment charges	$ 2	$ 11	$ 300
Gain (loss) associated with Maine fossil	$ 0	21	(67)
NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Capacity of Spain Solar projects - in megawatts | MW	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) | MW	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