Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Condensed Income Statements, Captions [Line Items]
Operating revenues	$ 1,112,777		$ 733,649
Gas purchases	271,120		179,956
Operating expenses	100,153		64,224
General and administrative expenses	56,387		37,215
Depreciation, depletion and amortization	225,076		179,467
Taxes, other than income taxes	25,422		20,827
Total operating costs and expenses	678,158		481,689
Operating income	434,619		251,960	[1]
Other loss, net	1,193		(533)
Loss on derivatives	(99,720)		(29,794)
Interest expense	12,907	[1]	9,021	[1]
Income (loss) before income taxes	323,185		212,612
Provision for income taxes	128,995	[1]	85,097	[1]
Net income	194,190		127,515
Comprehensive income	163,565		33,318
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			(77,864)
Gas purchases			(208)
Operating expenses			(77,639)
General and administrative expenses			(17)
Total operating costs and expenses			(77,864)
Equity in earnings of subsidiaries			(127,515)
Income (loss) before income taxes			(127,515)
Net income			(127,515)
Comprehensive income			(33,051)
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Equity in earnings of subsidiaries			127,515
Income (loss) before income taxes			127,515
Net income			127,515
Comprehensive income			33,318
Former Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			690,144
Gas purchases			180,164
Operating expenses			110,541
General and administrative expenses			31,866
Depreciation, depletion and amortization			167,522
Taxes, other than income taxes			17,715
Total operating costs and expenses			507,808
Operating income			182,336
Other loss, net			(376)
Loss on derivatives			(29,794)
Interest expense			6,938
Income (loss) before income taxes			145,228
Provision for income taxes			58,046
Net income			87,182
Comprehensive income			(6,253)
Other Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			121,369
Operating expenses			31,322
General and administrative expenses			5,366
Depreciation, depletion and amortization			11,945
Taxes, other than income taxes			3,112
Total operating costs and expenses			51,745
Operating income			69,624
Other loss, net			(157)
Interest expense			2,083
Income (loss) before income taxes			67,384
Provision for income taxes			27,051
Net income			40,333
Comprehensive income			$ 39,304
7.35% Senior Notes due 2017 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.35%
7.125% Senior Notes due 2017 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.125%
7.15% Senior Notes due 2018 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.15%
7.5% Senior Notes Due 2018 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.50%
4.10% Senior Notes Due 2022 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	4.10%

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