Regulatory Matters - Southern Company Gas (Details) $ in Millions							1 Months Ended		4 Months Ended	12 Months Ended
	Oct. 08, 2019 USD ($)	Oct. 02, 2019	Sep. 25, 2019 USD ($)	Jun. 03, 2019 USD ($)	Apr. 16, 2019 USD ($) intervenor	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2017
Nicor Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount								$ 230
Public utilities, approved return on equity percentage					9.86%			10.60%
Regulatory equity ratio					54.00%			54.00%		52.00%
Number of intervenors | intervenor					2
Requested increase in rate amounts					$ 180
Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage				10.75%
Regulatory equity ratio				55.00%
Requested increase in rate amounts				$ 96
Requested increase in rate amounts, amended									$ 93
Virginia Natural Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount							$ (14)
Bill credits						$ 14
Virginia Natural Gas | SAVE
Loss Contingencies [Line Items]
Approved investment amount			$ 365
Infrastructure replacement program, approved investment variance amount			$ 5
Infrastructure replacement program, approved investment term			6 years
Virginia Natural Gas | SAVE | 2018
Loss Contingencies [Line Items]
Approved investment amount			$ 35
Virginia Natural Gas | SAVE | 2019
Loss Contingencies [Line Items]
Approved investment amount			40
Virginia Natural Gas | SAVE | 2020
Loss Contingencies [Line Items]
Approved investment amount			50
Virginia Natural Gas | SAVE | 2021
Loss Contingencies [Line Items]
Approved investment amount			60
Virginia Natural Gas | SAVE | 2022
Loss Contingencies [Line Items]
Approved investment amount			70
Virginia Natural Gas | SAVE | 2023
Loss Contingencies [Line Items]
Approved investment amount			70
Infrastructure replacement program, approved investment variance amount			10
Virginia Natural Gas | SAVE | 2024
Loss Contingencies [Line Items]
Approved investment amount			70
Infrastructure replacement program, approved investment variance amount			$ 10
Minimum | Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage				10.55%
Maximum | Atlanta Gas Light
Loss Contingencies [Line Items]
Public utilities, approved return on equity percentage				10.95%
Subsequent Event | Nicor Gas
Loss Contingencies [Line Items]
Approved increase (decrease) in fuel rates amount	$ 168
Public utilities, approved rate increase, recovery of investment, amount	$ 65
Public utilities, approved return on equity percentage	9.73%
Regulatory equity ratio	54.20%
Rehearing request period		30 days