Reportable Business Segments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended										12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Segment Reporting Information [Line Items]
Revenue from External Customers											$ 4,901		$ 6,102		$ 6,569
Inter-segment Revenues											0		0		0
Depreciation and Amortization											285		262		248
Operating Income (Loss)	211		(152)	106	229	177		111	114	260	394		662		667
Total Assets	11,190					10,687					11,190		10,687		10,538
Expenditures for Long-Lived Assets											549		599		974
Equity income											31		30		29
Retail gas sales											3,328		4,019		4,412
Wholesale gas sales											613		1,149		1,250
Gas transportation and processing											847		824		785
Energy products and services											113		110		122
Total	1,551		954	846	1,550	1,608		1,171	1,228	2,095	4,901		6,102		6,569
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											2,320		2,823		3,199
Inter-segment Revenues											22		18		14
Depreciation and Amortization											173		166		166
Operating Income (Loss)											226		226		231
Total Assets	4,775					4,636					4,775		4,636		4,575
Expenditures for Long-Lived Assets											359		295		202
Competitive Natural Gas Sales and Services [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											1,758		2,488		2,617
Inter-segment Revenues											26		23		34
Depreciation and Amortization											6		5		4
Operating Income (Loss)											(250)		6		16
Total Assets	839					1,089					839		1,089		1,190
Expenditures for Long-Lived Assets											6		5		2
Interstate Pipelines [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											356	[1]	421	[1]	464	[1]
Inter-segment Revenues											146	[1]	132	[1]	137	[1]
Depreciation and Amortization											56	[1]	54	[1]	52	[1]
Operating Income (Loss)											207	[1]	248	[1]	270	[1]
Total Assets	4,004	[1]				3,867	[1]				4,004	[1]	3,867	[1]	3,672	[1]
Expenditures for Long-Lived Assets											132	[1]	98	[1]	102	[1]
Equity income											26		21		19
Interest in jointly-owned asset (in hundredths)	50.00%										50.00%
Investment in unconsolidated affiliate	404					409					404		409		413
Field Services [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											467	[2]	370	[2]	289	[2]
Inter-segment Revenues											39	[2]	42	[2]	49	[2]
Depreciation and Amortization											50	[2]	37	[2]	25	[2]
Operating Income (Loss)											214	[2]	189	[2]	151	[2]
Total Assets	2,453	[2]				1,894	[2]				2,453	[2]	1,894	[2]	1,803	[2]
Expenditures for Long-Lived Assets											52	[2]	201	[2]	668	[2]
Equity income											5		9		10
Interest in jointly-owned asset (in hundredths)	50.00%										50.00%
Investment in unconsolidated affiliate						63							63		55
Other [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											0		0		0
Inter-segment Revenues											0		0		0
Depreciation and Amortization											0		0		1
Operating Income (Loss)											(3)		(7)		(1)
Total Assets	647					660					647		660		659
Expenditures for Long-Lived Assets											0		0		0
Reconciling Eliminations [Member]
Segment Reporting Information [Line Items]
Revenue from External Customers											0		0		0
Inter-segment Revenues											(233)		(215)		(234)
Depreciation and Amortization											0		0		0
Operating Income (Loss)											0		0		0
Total Assets	(1,528)					(1,459)					(1,528)		(1,459)		(1,361)
Expenditures for Long-Lived Assets											$ 0		$ 0		$ 0

[1] Interstate Pipelines recorded equity income of $19 million, $21 million and $26 million in the years ended December 31, 2010, 2011 and 2012, respectively, from its 50% interest in SESH, a jointly-owned pipeline. These amounts are included in Equity in earnings of unconsolidated affiliates under the Other Income (Expense) caption. Interstate Pipelines’ investment in SESH was $413 million, $409 million and $404 million as of December 31, 2010, 2011 and 2012, respectively, and is included in Investment in unconsolidated affiliates.

[2] Field Services recorded equity income of $10 million, $9 million and $5 million for the years ended December 31, 2010, 2011 and 2012, respectively, from its 50% interest in a jointly-owned gas processing plant. These amounts are included in Equity in earnings of unconsolidated affiliates under the Other Income (Expense) caption. Field Services’ investment in the jointly-owned gas processing plant was $55 million and $63 million as of December 31, 2010 and 2011, respectively, and is included in Investment in unconsolidated affiliates. Beginning on August 1, 2012, financial results for Waskom are included in operating income due to the July 31, 2012 purchase of the 50% interest in Waskom that CERC did not already own.