Acquisitions and Dispositions - Schedule of Construction Projects (Details) - SOUTHERN POWER CO - MW		1 Months Ended		3 Months Ended
	Jan. 06, 2017	Apr. 30, 2017	Mar. 31, 2017	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Jul. 31, 2017
Bethel
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	276
Southern Power Percentage Ownership	100.00%
PPA Contract Period	12 years
East Pecos
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)						120
PPA Contract Period			15 years
Lamesa
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)						102
PPA Contract Period		15 years
Cactus Flats
Business Acquisition [Line Items]
Southern Power Percentage Ownership							100.00%
Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)					148
Scenario, Forecast | Mankato
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)				345
PPA Contract Period				20 years
Minimum | Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period					12 years
Maximum | Scenario, Forecast | Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period					15 years