Summary of Significant Accounting Policies - Concentration of Revenue (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Jan. 29, 2019 Facility MW
Concentration Risk [Line Items]
Outstanding accounts receivable	$ 2,630
Mississippi Power
Concentration Risk [Line Items]
Wholesale customers representing total operating revenues (percent)	17.30%
Period of cancellation notices	10 years
Outstanding accounts receivable	$ 100
Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	118
Subsequent Event | PG&E
Concentration Risk [Line Items]
Number of solar facilities | Facility		4
Capacity of natural gas facility (mw) | MW		207
PPAs | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	1
Transmission Interconnections | Southern Power
Concentration Risk [Line Items]
Outstanding accounts receivable	36
Outstanding accounts receivable classified in other deferred charges and assets	$ 17