Reportable Business Segments (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues from external customers		$ 2,093	$ 1,320
Operating Income (Loss)		194	166
Assets		9,357		$ 9,218
Equity in earnings of unconsolidated affiliate, net		72	60
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		907	888
Operating Income (Loss)		164	160
Energy Services [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		1,185	432
Operating Income (Loss)		35	6
Midstream Investments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	[1]	0	0
Operating Income (Loss)	[1]	0	0
Other Operations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		1	0
Operating Income (Loss)		(5)	0
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(20)	(14)
Assets		(521)		(563)
Intersegment Eliminations [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(9)	(7)
Intersegment Eliminations [Member] | Energy Services [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		(11)	(7)
Intersegment Eliminations [Member] | Midstream Investments [Member]
Segment Reporting Information [Line Items]
Revenues from external customers	[1]	0	0
Intersegment Eliminations [Member] | Other Operations [Member]
Segment Reporting Information [Line Items]
Revenues from external customers		0	0
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Assets		5,975		6,099
Operating Segments [Member] | Energy Services [Member]
Segment Reporting Information [Line Items]
Assets		1,326		1,102
Operating Segments [Member] | Midstream Investments [Member]
Segment Reporting Information [Line Items]
Assets	[1]	2,502		2,505
Operating Segments [Member] | Other Operations [Member]
Segment Reporting Information [Line Items]
Assets		75		$ 75
Enable Midstream Partners [Member]
Segment Reporting Information [Line Items]
Equity in earnings of unconsolidated affiliate, net		72	60
Enable Midstream Partners [Member] | Midstream Investments [Member]
Segment Reporting Information [Line Items]
Equity in earnings of unconsolidated affiliate, net		$ 72	$ 60

[1] Midstream Investments’ equity earnings are as follows: Three Months Ended March 31, 2017 2016 (in millions)Enable $72 $60