Acquisitions and Dispositions - Construction Projects (Details) - Southern Power - MW		1 Months Ended	3 Months Ended
	Jul. 31, 2017	Jul. 31, 2018	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Aug. 31, 2018	May 31, 2018
Cactus Flats
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)	148	148
Minimum \| Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period	12 years	12 years
Maximum \| Cactus Flats
Business Acquisition [Line Items]
PPA Contract Period	15 years	15 years
Cactus Flats \| Cactus Flats
Business Acquisition [Line Items]
Ownership Percentage	100.00%
Cactus Flats \| Class B Membership Interest \| Cactus Flats
Business Acquisition [Line Items]
Ownership Percentage						100.00%
Wild Horse Mountain \| Wild Horse Mountain
Business Acquisition [Line Items]
Ownership Percentage							100.00%
Wild Horse Mountain \| Class B Membership Interest \| Wild Horse Mountain
Business Acquisition [Line Items]
Ownership Percentage							100.00%
Reading \| Reading
Business Acquisition [Line Items]
Ownership Percentage						100.00%
Reading \| Class B Membership Interest \| Reading
Business Acquisition [Line Items]
Ownership Percentage						100.00%
Scenario, Forecast \| Mankato Expansion
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)					385
PPA Contract Period					20 years
Scenario, Forecast \| Wild Horse Mountain
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)				100
PPA Contract Period				20 years
Scenario, Forecast \| Reading
Business Acquisition [Line Items]
Approximate Nameplate Capacity (MW)			200
PPA Contract Period			12 years