Regulatory Matters - Southern Company Gas (Details) $ in Millions				1 Months Ended		12 Months Ended
	Apr. 16, 2019 USD ($) intervenor	Apr. 08, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($)	Dec. 31, 2018
Nicor Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount					$ 230
Public utilities, approved return on equity percentage					10.60%
Regulatory equity ratio					54.00%	52.00%
Virginia Natural Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount				$ (14)
Bill credits			$ 14
Subsequent Event | Nicor Gas
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage	9.86%
Regulatory equity ratio	54.00%
Number of intervenors | intervenor	2
Requested increase in rate amounts	$ 180
Subsequent Event | SAVE | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		$ 370
Infrastructure replacement program, petitioned investment renewal term		5 years
Infrastructure replacement program, petitioned investment term		6 years
Infrastructure replacement program, proposed investment variance amount		$ 10
Subsequent Event | SAVE | 2018 | Virginia Natural Gas
Loss Contingencies [Line Items]
Approved investment amount		35
Subsequent Event | SAVE | 2019 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		40
Infrastructure replacement program, proposed investment variance amount		6
Subsequent Event | SAVE | 2020 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		50
Infrastructure replacement program, proposed investment variance amount		8
Subsequent Event | SAVE | 2021 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		60
Infrastructure replacement program, proposed investment variance amount		9
Subsequent Event | SAVE | 2022 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		70
Infrastructure replacement program, proposed investment variance amount		10
Subsequent Event | SAVE | 2023 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		70
Infrastructure replacement program, proposed investment variance amount		10
Subsequent Event | SAVE | 2024 | Virginia Natural Gas
Loss Contingencies [Line Items]
Infrastructure replacement program, proposed investment amount		70
Infrastructure replacement program, proposed investment variance amount		$ 10