Fair Value Measurements (Nonrecurring Fair Value Measurements) (Details) $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2019	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)	[2],[3]	Dec. 31, 2015 pipeline
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of long-term contracted natural gas pipeline assets | pipeline							7
Long-lived assets, carrying amount		$ 70,334	$ 72,289	[1]
Impairment of long-lived assets		$ 11	446	[2],[3]	$ 7
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 Plant 2 Member | NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Proportionate ownership share		70.00%	70.00%
Subsequent Event [Member] | Purchased Power Agreements [Member] | NEER [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shortened term	5 years

[1] Amounts have been retrospectively adjusted as discussed in Note 14.

[2] Amounts have been retrospectively adjusted as discussed in Note 14 and Note 3 - Amendments to Presentation of Retirement Benefits.

[3] Amounts have been retrospectively adjusted as discussed in Note 14.