Leases - Undiscounted Cash Flows to be Received Under PPAs Accounted for as Operating Leases (Details) $ in Millions	Mar. 31, 2019 USD ($)
Lessor, Lease, Description [Line Items]
2019 (remaining)	$ 163
2020	188
2021	183
2022	174
2023	171
Thereafter	1,809
Total	2,688
GEORGIA POWER CO
Lessor, Lease, Description [Line Items]
2019 (remaining)	20
2020	26
2021	18
2022	8
2023	2
Thereafter	0
Total	74
SOUTHERN POWER CO
Lessor, Lease, Description [Line Items]
2019 (remaining)	123
2020	128
2021	131
2022	134
2023	137
Thereafter	1,017
Total	1,670
SOUTHERN Co GAS
Lessor, Lease, Description [Line Items]
2019 (remaining)	26
2020	34
2021	34
2022	34
2023	34
Thereafter	498
Total	$ 660