Segment Information - Intersegment Reporting (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenues	$ 571	$ 674	$ 1,905	$ 2,395
Operating expenses
Cost of goods sold	191	222	769	1,157
Operation and maintenance	213	209	454	458
Depreciation and amortization	104	98	206	195
Taxes other than income taxes	37	38	99	114
Merger-related expenses	53	0	56	0
Total operating expenses	598	567	1,584	1,924
Operating (loss) income	(27)	107	321	471
Other income	3	4	6	7
EBIT	(24)	111	327	478
Total assets	14,488	13,835	14,488	13,835	$ 14,754
Capital expenditures	313	264	548	452
Intercompany Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(42)	(43)	(102)	(119)
Operating expenses
Cost of goods sold	(41)	(41)	(98)	(116)
Operation and maintenance	(2)	(2)	(4)	(3)
Depreciation and amortization	0	0	0	0
Taxes other than income taxes	0	0	0	0
Merger-related expenses	0		0
Total operating expenses	(43)	(43)	(102)	(119)
Operating (loss) income	1	0	0	0
Other income	0	0	0	0
EBIT	1	0	0	0
Total assets	(9,673)	(9,279)	(9,673)	(9,279)	(9,740)
Capital expenditures	0	0	0	0
Distribution Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	547	546	1,575	1,887
Operating expenses
Cost of goods sold	144	166	608	942
Operation and maintenance	164	160	349	345
Depreciation and amortization	89	83	178	165
Taxes other than income taxes	33	34	89	105
Merger-related expenses	0		0
Total operating expenses	430	443	1,224	1,557
Operating (loss) income	117	103	351	330
Other income	2	3	2	4
EBIT	119	106	353	334
Total assets	12,499	11,796	12,499	11,796	12,517
Capital expenditures	280	248	484	418
Retail Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	149	153	435	494
Operating expenses
Cost of goods sold	83	95	245	305
Operation and maintenance	31	34	68	71
Depreciation and amortization	5	6	11	12
Taxes other than income taxes	1	1	2	2
Merger-related expenses	0		0
Total operating expenses	120	136	326	390
Operating (loss) income	29	17	109	104
Other income	0	0	0	0
EBIT	29	17	109	104
Total assets	672	636	672	636	686
Capital expenditures	2	2	4	4
Wholesale Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	(95)	4	(32)	94
Operating expenses
Cost of goods sold	1	0	4	9
Operation and maintenance	14	11	30	35
Depreciation and amortization	1	1	1	1
Taxes other than income taxes	1	0	2	1
Merger-related expenses	0		0
Total operating expenses	17	12	37	46
Operating (loss) income	(112)	(8)	(69)	48
Other income	0	0	1	0
EBIT	(112)	(8)	(68)	48
Total assets	753	806	753	806	935
Capital expenditures	1	0	1	1
Midstream Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	10	11	25	30
Operating expenses
Cost of goods sold	4	2	10	12
Operation and maintenance	6	6	13	12
Depreciation and amortization	5	4	9	9
Taxes other than income taxes	1	1	2	2
Merger-related expenses	0		0
Total operating expenses	16	13	34	35
Operating (loss) income	(6)	(2)	(9)	(5)
Other income	1	0	3	1
EBIT	(5)	(2)	(6)	(4)
Total assets	744	686	744	686	692
Capital expenditures	25	7	43	10
Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	2	3	4	9
Operating expenses
Cost of goods sold	0	0	0	5
Operation and maintenance	0	0	(2)	(2)
Depreciation and amortization	4	4	7	8
Taxes other than income taxes	1	2	4	4
Merger-related expenses	53		56
Total operating expenses	58	6	65	15
Operating (loss) income	(56)	(3)	(61)	(6)
Other income	0	1	0	2
EBIT	(56)	(2)	(61)	(4)
Total assets	9,493	9,190	9,493	9,190	$ 9,664
Capital expenditures	5	7	16	19
Reportable Subsegments [Member]
Segment Reporting Information [Line Items]
Operating revenues	571	674	1,905	2,395
Reportable Subsegments [Member] | Intercompany Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(4)	(3)	(19)	(23)
Reportable Subsegments [Member] | Distribution Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	509	506	1,492	1,791
Reportable Subsegments [Member] | Retail Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	149	153	435	494
Reportable Subsegments [Member] | Wholesale Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	(95)	4	(32)	94
Reportable Subsegments [Member] | Midstream Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	10	11	25	30
Reportable Subsegments [Member] | Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	2	3	4	9
Intercompany revenues [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Intercompany revenues [Member] | Intercompany Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	(38)	(40)	(83)	(96)
Intercompany revenues [Member] | Distribution Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	38	40	83	96
Intercompany revenues [Member] | Retail Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Intercompany revenues [Member] | Wholesale Services [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Intercompany revenues [Member] | Midstream Operations [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Intercompany revenues [Member] | Other [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	$ 0	$ 0	$ 0	$ 0