Segment Information (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Segment Information
Total segment gross margin	$ 64,842	$ 57,903	$ 190,484	$ 165,863
Operations and maintenance expenses	16,091	13,004	46,953	38,337
Depreciation, amortization and impairment	21,911	15,218	56,143	46,002
General and administrative expenses	10,031	9,869	34,530	31,311
Taxes other than income	1,502	1,315	4,029	3,658
Equity in loss (earnings) from unconsolidated affiliates	161,589	(2,049)	158,581	19,788
Operating (loss) income	(146,282)	20,546	(109,752)	26,767
Natural gas sales	120,815	87,524	348,538	292,559
Natural gas liquids sales	191,370	118,999	521,129	353,119
Transportation, compression and processing fees	30,337	17,909	82,706	47,539
Condensate and other	11,169	13,272	37,299	41,204
Sales to external customers	353,691	237,704	989,672	734,421
Interest and other financing costs	(11,080)	(12,943)	(34,450)	(41,239)
Total assets	2,017,900		2,017,900		1,906,993
Texas [Member]
Segment Information
Total segment gross margin	44,540	31,218	135,685	90,134
Operations and maintenance expenses	9,082	6,779	26,815	20,845
Depreciation, amortization and impairment	7,182	6,106	20,712	18,143
General and administrative expenses	3,010	2,432	8,731	7,606
Taxes other than income	711	597	1,938	1,860
Equity in loss (earnings) from unconsolidated affiliates	(1,894)	97	(1,698)	188
Operating (loss) income	26,449	15,207	79,187	41,492
Natural gas sales	74,681	41,906	211,466	142,339
Natural gas liquids sales	118,280	62,032	315,879	182,334
Transportation, compression and processing fees	23,407	11,263	61,989	29,288
Condensate and other	2,874	3,020	12,366	7,828
Sales to external customers	219,242	118,221	601,700	361,789
Total assets	807,512		807,512
Oklahoma (a) [Member]
Segment Information
Total segment gross margin	27,876	23,010	79,623	69,106
Operations and maintenance expenses	6,930	6,163	19,943	17,266
Depreciation, amortization and impairment	8,623	7,941	27,024	24,297
General and administrative expenses	2,199	2,361	6,766	6,338
Taxes other than income	774	702	2,056	1,779
Equity in loss (earnings) from unconsolidated affiliates	(652)	(671)	(2,023)	(2,311)
Operating (loss) income	10,002	6,514	25,857	21,737
Natural gas sales	47,528	46,958	141,264	153,661
Natural gas liquids sales	79,154	55,423	224,174	170,771
Transportation, compression and processing fees	2,727	2,229	7,927	4,860
Condensate and other	8,417	7,291	27,392	24,319
Sales to external customers	137,826	111,901	400,757	353,611
Total assets	671,611		671,611
Rocky Mountains [Member]
Segment Information
Total segment gross margin	432	1,091	2,245	3,342
Operations and maintenance expenses	79	62	195	226
Depreciation, amortization and impairment	5,764	765	7,295	2,296
General and administrative expenses	339	362	1,003	1,487
Taxes other than income	17	16	18	19
Equity in loss (earnings) from unconsolidated affiliates	164,135	(1,475)	162,302	21,911
Operating (loss) income	(169,902)	1,361	(168,568)	(22,597)
Natural gas sales	127	164	380	1,099
Transportation, compression and processing fees	4,203	4,417	12,790	13,391
Condensate and other	299	417	1,114	1,250
Sales to external customers	4,629	4,998	14,284	15,740
Total assets	456,402		456,402
Total Segments [Member]
Segment Information
Total segment gross margin	72,848	55,319	217,553	162,582
Operations and maintenance expenses	16,091	13,004	46,953	38,337
Depreciation, amortization and impairment	21,569	14,812	55,031	44,736
General and administrative expenses	5,548	5,155	16,500	15,431
Taxes other than income	1,502	1,315	4,012	3,658
Equity in loss (earnings) from unconsolidated affiliates	161,589	(2,049)	158,581	19,788
Operating (loss) income	(133,451)	23,082	(63,524)	40,632
Natural gas sales	122,336	89,028	353,110	297,099
Natural gas liquids sales	197,434	117,455	540,053	353,105
Transportation, compression and processing fees	30,337	17,909	82,706	47,539
Condensate and other	11,590	10,728	40,872	33,397
Sales to external customers	361,697	235,120	1,016,741	731,140
Total assets	1,935,525		1,935,525
Corporate and Other [Member]
Segment Information
Total segment gross margin	(8,006)	2,584	(27,069)	3,281
Depreciation, amortization and impairment	342	406	1,112	1,266
General and administrative expenses	4,483	4,714	18,030	15,880
Taxes other than income			17
Operating (loss) income	(12,831)	(2,536)	(46,228)	(13,865)
Natural gas sales	(1,521)	(1,504)	(4,572)	(4,540)
Natural gas liquids sales	(6,064)	1,544	(18,924)	14
Condensate and other	(421)	2,544	(3,573)	7,807
Sales to external customers	(8,006)	2,584	(27,069)	3,281
Interest and other financing costs	(11,080)	(12,943)	(34,450)	(41,239)
Total assets	$ 82,375		$ 82,375