Acquisitions and Dispositions - Southern Power Narrative (Details)	1 Months Ended	2 Months Ended	3 Months Ended			8 Months Ended	9 Months Ended		12 Months Ended	36 Months Ended
	May 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) operating_facility	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2019 USD ($) MW	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Construction work in progress				$ 7,655,000,000			$ 7,655,000,000				$ 6,904,000,000
Impairment charges				36,000,000	$ 0		197,000,000	$ 0
Increase (decrease) due to sale of noncontrolling interests							1,280,000,000
Assets subject to lien				111,717,000,000			111,717,000,000				111,005,000,000
Common Stockholders' Equity
Business Acquisition [Line Items]
Increase (decrease) due to sale of noncontrolling interests							(410,000,000)
SOUTHERN POWER CO
Business Acquisition [Line Items]
Construction work in progress				586,000,000			586,000,000				511,000,000
Impairment charges				36,000,000	$ 0		155,000,000	0
Repayments of debt	$ 770,000,000
Repayments of capital	250,000,000						650,000,000	$ 0
Increase (decrease) due to sale of noncontrolling interests	511,000,000
Assets subject to lien				15,355,000,000			15,355,000,000				$ 15,206,000,000
SOUTHERN POWER CO | Scenario, Forecast
Business Acquisition [Line Items]
Number of operating wind facilities | operating_facility			8
SOUTHERN POWER CO | Common Stockholders' Equity
Business Acquisition [Line Items]
Increase (decrease) due to sale of noncontrolling interests							(410,000,000)
SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Florida Plants
Business Acquisition [Line Items]
Impairment charges							36,000,000
Proceeds from sale of equity method investments	195,000,000
Decrease in consideration per day between end of fiscal year and closing	110,000
Increase in consideration per day if closing is prior to end of fiscal year	$ 110,000
Pre-tax impairment charge							119,000,000
After tax impairment charge							89,000,000
SOUTHERN POWER CO | Wind Generating Facility
Business Acquisition [Line Items]
Impairment charges				36,000,000			155,000,000
SOUTHERN POWER CO | Wind Generating Facility | Scenario, Forecast
Business Acquisition [Line Items]
Project qualification for production tax credits, percentage									80.00%	100.00%
SOUTHERN POWER CO | SP Solar Holdings I, LP
Business Acquisition [Line Items]
Limited liability company or limited partnership, members or limited partners, ownership interest	33.00%
Proceeds from sale of equity method investments	$ 1,200,000,000
SOUTHERN POWER CO | Series of Construction Projects
Business Acquisition [Line Items]
Construction work in progress				246,000,000			246,000,000
SOUTHERN POWER CO | Series of Construction Projects | Minimum
Business Acquisition [Line Items]
Estimated future construction payments				575,000,000			575,000,000
SOUTHERN POWER CO | Series of Construction Projects | Maximum
Business Acquisition [Line Items]
Estimated future construction payments				640,000,000			640,000,000
SOUTHERN POWER CO | Plant Mankato | Senior Lien
Business Acquisition [Line Items]
Assets subject to lien				$ 500,000,000			$ 500,000,000
SOUTHERN POWER CO | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Expansion currently under construction (MW) | MW						385
Tax Equity Funding | SOUTHERN POWER CO | Scenario, Forecast
Business Acquisition [Line Items]
Repayments of capital		$ 1,000,000,000
Tax equity funding		$ 1,200,000,000	$ 1,200,000,000
Subsequent Event | SOUTHERN POWER CO | Disposal Group, Held-for-sale, Not Discontinued Operations | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Proceeds from sale of equity method investments						$ 650,000,000
Decrease per day after June 1, 2019						66,667
Maximum price decrease						$ 15,000,000
Subsequent Event | SOUTHERN POWER CO | Plant Mankato | Scenario, Forecast
Business Acquisition [Line Items]
Expansion currently under construction (MW) | MW						385