Contingencies and Regulatory Matters - Table - Unrecognized Ratemaking Amounts (Details) - Regulatory Asset Off Balance Sheet - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Successor [Member]
Loss Contingencies [Line Items]
Regulatory asset	$ 129
Successor [Member] | Atlanta Gas Light
Loss Contingencies [Line Items]
Regulatory asset	110
Successor [Member] | Virginia Natural Gas
Loss Contingencies [Line Items]
Regulatory asset	11
Successor [Member] | Elizabethtown Gas [Member]
Loss Contingencies [Line Items]
Regulatory asset	6
Successor [Member] | Nicor Gas [Member]
Loss Contingencies [Line Items]
Regulatory asset	$ 2
Predecessor [Member]
Loss Contingencies [Line Items]
Regulatory asset		$ 122
Predecessor [Member] | Atlanta Gas Light
Loss Contingencies [Line Items]
Regulatory asset		103
Predecessor [Member] | Virginia Natural Gas
Loss Contingencies [Line Items]
Regulatory asset		12
Predecessor [Member] | Elizabethtown Gas [Member]
Loss Contingencies [Line Items]
Regulatory asset		4
Predecessor [Member] | Nicor Gas [Member]
Loss Contingencies [Line Items]
Regulatory asset		$ 3