Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Operating Revenues	$ 733,649		$ 654,835
Gas purchases - midstream services	179,956		146,676
Operating expenses	64,224		60,958
General and administrative expenses	37,215		48,826
Depreciation, depletion and amortization	179,467		193,627
Taxes, other than income taxes	20,827		20,422
Total operating costs and expenses	481,689		470,509
Operating income	251,960		184,326
Other income (loss), net	(533)		(200)
Commodity derivative gain (loss)	(29,794)		1,634
Interest expense	9,021	[1]	7,338	[1]
Income Before Income Taxes	212,612		178,422
Provision for income taxes	85,097	[1]	70,718	[1]
Net income	127,515		107,704
Comprehensive income	33,318		174,274
Parent [Member]
Equity in earnings of subsidiaries	127,515		107,704
Income Before Income Taxes	127,515		107,704
Net income	127,515		107,704
Comprehensive income	33,318		107,958
Guarantors [Member]
Operating Revenues	690,144		612,811
Gas purchases - midstream services	180,164		146,903
Operating expenses	110,541		103,056
General and administrative expenses	31,866		41,708
Depreciation, depletion and amortization	167,522		182,837
Taxes, other than income taxes	17,715		16,998
Total operating costs and expenses	507,808		491,502
Operating income	182,336		121,309
Other income (loss), net	(376)		(174)
Commodity derivative gain (loss)	(29,794)		1,634
Interest expense	6,938		3,755
Income Before Income Taxes	145,228		119,014
Provision for income taxes	58,046		46,892
Net income	87,182		72,122
Comprehensive income	(6,253)		137,957
Non-Guarantors [Member]
Operating Revenues	121,369		113,125
Operating expenses	31,322		27,981
General and administrative expenses	5,366		7,913
Depreciation, depletion and amortization	11,945		10,790
Taxes, other than income taxes	3,112		3,424
Total operating costs and expenses	51,745		50,108
Operating income	69,624		63,017
Other income (loss), net	(157)		(26)
Interest expense	2,083		3,583
Income Before Income Taxes	67,384		59,408
Provision for income taxes	27,051		23,826
Net income	40,333		35,582
Comprehensive income	39,304		36,063
Eliminations [Member]
Operating Revenues	(77,864)		(71,101)
Gas purchases - midstream services	(208)		(227)
Operating expenses	(77,639)		(70,079)
General and administrative expenses	(17)		(795)
Total operating costs and expenses	(77,864)		(71,101)
Equity in earnings of subsidiaries	(127,515)		(107,704)
Income Before Income Taxes	(127,515)		(107,704)
Net income	(127,515)		(107,704)
Comprehensive income	$ (33,051)		$ (107,704)

[1] Interest expense and the provision for income taxes by segment are allocated as they are incurred at the corporate level.