Property, Plant and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 5,898	$ 5,364
Accumulated depreciation and amortization	1,640	1,554
Property, Plant and Equipment, Net	4,258	3,810
Depreciation expense	211	195	$ 218
Amortization expense	16	11	12
Total depreciation and amortization expense	227	206	230
Beginning balance	139	101
Accretion expense	5	4
Revisions in estimates of cash flows	12	34
Ending balance	156	139	$ 101
Natural Gas Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	5,762	5,235
Accumulated depreciation and amortization	$ 1,575	1,493
Natural Gas Distribution [Member] | Weighted Average [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	32 years
Energy Services [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 86	84
Accumulated depreciation and amortization	$ 34	31
Energy Services [Member] | Weighted Average [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	27 years
Other property [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Gross	$ 50	45
Accumulated depreciation and amortization	$ 31	$ 30
Other property [Member] | Weighted Average [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years