Regulated Operations (Details) $ in Millions	1 Months Ended	3 Months Ended
	Oct. 31, 2015 USD ($)	Mar. 31, 2016 USD ($) mi	Jun. 30, 2016 USD ($) site	Dec. 31, 2015 USD ($)	Jun. 30, 2015 USD ($)	Feb. 28, 2015 USD ($)
Regulated Operations [Line Items]
Regulatory assets – current			$ 47	$ 68	$ 48
Number of former operating sites | site			46
Recoverable ERC Assets [Member]
Regulated Operations [Line Items]
Regulatory assets – current			$ 18
Nicor Gas [Member]
Regulated Operations [Line Items]
Number of former operating sites | site			26
Number of sites with completed cleanups | site			2
Number of sites with cleanup responsibility | site			23
Cleanup costs allocated (as a percent)			52.00%
True-up Recovery [Member] | Atlanta Gas Light [Member]
Regulated Operations [Line Items]
Allowable amount of costs incurred to be recovered	$ 144
Unrecovered program revenue to be recovered						$ 178
SAVE Program [Member] | Virginia Natural Gas [Member]
Regulated Operations [Line Items]
Natural gas pipeline, approved for replacement, more than (in miles) | mi		200
Approved infrastructure replacement program amount, 2016		$ 30
Approved infrastructure replacement program amount, 2017		35
Approved infrastructure replacement program amount, 2018		35
Approved infrastructure replacement program amount, 2019		35
Approved infrastructure replacement program amount, 2020		35
Approved infrastructure replacement program amount, 2021		$ 35