Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets, carrying amount	$ 72,416	$ 66,912
Impairment of long-lived assets	446	$ 7	$ 2
NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-lived assets, carrying amount	502
Fair value of long-lived assets	82
Impairment of long-lived assets	420
Impairment of long-lived assets, after tax	$ 258
Jointly Owned Nuclear Power Plant2 Member | NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proportionate ownership share	70.00%