Contingencies and Regulatory Matters - Table - Gas (Details) - Regulatory Asset Off Balance Sheet - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 125
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	104
Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	11
Elizabethtown Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	8
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset	$ 2
Predecessor
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset		$ 129
Predecessor | Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset		110
Predecessor | Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset		11
Predecessor | Elizabethtown Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset		6
Predecessor | Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Regulatory asset		$ 2