Regulatory Matters - GAS Regulatory Matters (Details) $ in Millions										1 Months Ended			3 Months Ended	12 Months Ended							48 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 17, 2018 USD ($)	Nov. 09, 2018 USD ($)	May 15, 2018	May 05, 2018 USD ($)	Feb. 23, 2018 USD ($)	Feb. 08, 2018 USD ($)	Dec. 21, 2017 USD ($)	Oct. 10, 2017 USD ($)	Oct. 31, 2018 USD ($)	Jul. 31, 2018 USD ($)	Feb. 28, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) mi	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2017 USD ($)	Jan. 01, 2018 USD ($)
Nicor Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount			$ (230)		$ (44)		$ 137
Public utilities, approved return on equity			10.60%		9.80%		9.80%
Regulatory equity ratio			54.00%												52.00%
Nicor Gas | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Infrastructure investment, annual customer rate increase																			4.00%
Infrastructure investment, annual customer rate increase in any given year																			5.50%
Nicor Gas | Regulatory Infrastructure Program
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																		9 years
Nicor Gas | Investing In Illinois
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease) amount recovery of investments							$ 93
Nicor Gas | energySMART
Schedule of Regulatory Assets and Liabilities [Line Items]
Energy efficiency and term reduction expenditures														$ 29							$ 107
Approved infrastructure replacement program																		$ 113				$ 160
Virginia Natural Gas
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount		$ (14)						$ 34
Public utilities, approved return on equity								9.50%
Contract with customer, liability, bill credits	$ 14
Virginia Natural Gas | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity								9.00%
Virginia Natural Gas | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity								10.00%
Virginia Natural Gas | SAVE
Schedule of Regulatory Assets and Liabilities [Line Items]
Program duration period																				5 years
Regulated operations, natural gas pipeline length, approved for replacement | mi																200
Public utilities, approved rate increase (decrease) amount recovery of investments								$ 13
Virginia Natural Gas | Year 2016 | SAVE | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year																$ 30
Virginia Natural Gas | Year 2017 through 2021 | SAVE | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Approved infrastructure replacement program, approved investment amount, current fiscal year	35													35
Atlanta Gas Light
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved rate increase (decrease), amount						$ 16						$ 20
Public utilities, approved return on equity												10.75%
Annual proceeds from strategic economic development projects									$ 15
Regulatory equity ratio				55.00%
Contract with customer, liability, bill credits										$ 8	$ 8
Public utilities approved rate increase (decrease)				4.00%
Atlanta Gas Light | Minimum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity												10.55%			10.55%
Atlanta Gas Light | Maximum
Schedule of Regulatory Assets and Liabilities [Line Items]
Public utilities, approved return on equity												10.95%			10.95%
Atlanta Gas Light | Favorable Regulatory Action
Schedule of Regulatory Assets and Liabilities [Line Items]
Litigation settlement																	$ 144
Proceeds from legal settlements													$ 7		$ 20
Atlanta Gas Light | True Up Recovery
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount	171													171
Atlanta Gas Light | True-Up Recovery, Unrecognized Equity
Schedule of Regulatory Assets and Liabilities [Line Items]
Gain contingency, unrecorded amount	$ 95													$ 95