Property, Plant and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 6,888	$ 6,351
Accumulated depreciation and amortization	2,036	1,782
Property, plant and equipment, net	4,852	4,569
Depreciation expense	243	230	$ 211
Amortization expense	36	19	16
Total depreciation and amortization expense	279	249	227
Beginning balance	169	156
Accretion expense	7	8
Revisions in estimates of cash flows	67	5
Ending balance	243	169	$ 156
Natural Gas Distribution [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	6,735	6,219
Accumulated depreciation and amortization	$ 1,968	1,722
Natural Gas Distribution [Member] | Weighted Average [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life	28 years
Energy Services [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 102	83
Accumulated depreciation and amortization	$ 35	29
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	$ 51	49
Accumulated depreciation and amortization	$ 33	$ 31
Other property [Member] | Weighted Average [Member]
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life	14 years