CONTINGENCIES, COMMITMENTS, AND GUARANTEES - GAS Other Matters (Details)		12 Months Ended
	Dec. 30, 2019	Dec. 31, 2019 USD ($) facility cavern pipeline	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 30, 2020 phase mi
Loss Contingencies [Line Items]
Impairment charges		$ 168,000,000	$ 210,000,000	$ 0
Southern Company Gas
Loss Contingencies [Line Items]
Impairment of long-lived assets		$ 0
Pipelines | Southern Company Gas
Loss Contingencies [Line Items]
Number of gas construction projects | pipeline		2
PennEast Pipeline
Loss Contingencies [Line Items]
Extension request, term	2 years
Maximum | Pipelines
Loss Contingencies [Line Items]
Construction contingency estimate		$ 8,000,000,000.0
Maximum | Pipelines | Southern Company Gas
Loss Contingencies [Line Items]
Construction contingency estimate		400,000,000
Maximum | PennEast Pipeline | Southern Company Gas
Loss Contingencies [Line Items]
Construction contingency estimate		$ 300,000,000
Natural Gas Storage - Salt Dome Caverns
Loss Contingencies [Line Items]
Number of salt dome caverns | cavern		2
Natural Gas Storage Facility | Southern Company Gas
Loss Contingencies [Line Items]
Impairment charges		$ 91,000,000
After tax impairment charges		69,000,000
Public utilities, property, plant and equipment, net		$ 326,000,000
Subsequent Event | PennEast Pipeline
Loss Contingencies [Line Items]
Construction amendment, number of phases | phase					2
Construction amendment, number of miles of pipe, phase one | mi					68
California and Texas | Natural Gas Storage Facility | Southern Company Gas
Loss Contingencies [Line Items]
Public utilities, number of facilities | facility		2