Merger with Southern Company (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Jul. 01, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Business Acquisition [Line Items]
Merger-related expenses		$ 53	$ 0	$ 56	$ 0
Common stock, shares authorized		750,000,000	750,000,000	750,000,000	750,000,000	750,000,000
Common stock, par value (in dollars per share)		$ 5	$ 5	$ 5	$ 5	$ 5
Southern Company [Member]
Business Acquisition [Line Items]
Merger-related expenses		$ 53		$ 56
Merger related costs, net of tax		39		41
Merger related costs, compensation related expenses		24		25
Financial Advisory, Legal, and Other Merger Related Costs [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Merger-related expenses		29		31
Subsequent Event [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Common stock right to convert to cash, price per share (in dollars per share)	$ 66
Conversion ratio, percentage of underlying stock award units (as a percent)	125.00%
Regulatory approval requirements, required rate credit payments to customers	$ 18
Shares, authorized	110,000,000
Common stock, shares authorized	100,000,000
Preferred stock, shares authorized	10,000,000
Common stock, par value (in dollars per share)	$ 0.01
Preferred stock, par value (in dollars per share)	$ 0.01
Callable Debt [Member]
Business Acquisition [Line Items]
Callable long-term debt reclassified to current debt		$ 275		$ 275
Elizabethtown Gas [Member] | Subsequent Event [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Regulatory approval requirements, base rate case filing period	3 years
Elkton Gas [Member] | Subsequent Event [Member] | Southern Company [Member]
Business Acquisition [Line Items]
Regulatory approval requirements, base rate case filing period	2 years