Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $)	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Condensed Income Statements, Captions [Line Items]
Operating revenues	$ 1,035,000,000		$ 862,000,000		$ 2,148,000,000		$ 1,595,000,000
Gas purchases	261,000,000		200,000,000		532,000,000		380,000,000
Operating expenses	101,000,000		82,000,000		201,000,000		146,000,000
General and administrative expenses	52,000,000		48,000,000		108,000,000		85,000,000
Depreciation, depletion and amortization	230,000,000		187,000,000		455,000,000		366,000,000
Taxes, other than income taxes	24,000,000		20,000,000		50,000,000		41,000,000
Total operating costs and expenses	668,000,000		537,000,000		1,346,000,000		1,018,000,000
Operating income (loss)	367,000,000		325,000,000		802,000,000		577,000,000
Other gain, net			1,000,000		1,000,000
Gain (loss) on derivatives	(8,000,000)		93,000,000		(108,000,000)		64,000,000
Interest expense	12,000,000	[1]	9,000,000	[1]	25,000,000	[1]	19,000,000	[1]
Income (loss) before income taxes	347,000,000		410,000,000		670,000,000		622,000,000
Provision for income taxes	140,000,000	[1]	164,000,000	[1]	269,000,000	[1]	249,000,000	[1]
Net income	207,000,000		246,000,000		401,000,000		373,000,000
Comprehensive income (loss)	230,000,000		286,000,000		393,000,000		320,000,000
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			(83,000,000)				(161,000,000)
Gas purchases							(1,000,000)
Operating expenses			(83,000,000)				(160,000,000)
Total operating costs and expenses			(83,000,000)				(161,000,000)
Equity in earnings of subsidiaries			(246,000,000)				(373,000,000)
Income (loss) before income taxes			(246,000,000)				(373,000,000)
Net income			(246,000,000)				(373,000,000)
Comprehensive income (loss)			(285,000,000)				(318,000,000)
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Equity in earnings of subsidiaries			246,000,000				373,000,000
Income (loss) before income taxes			246,000,000				373,000,000
Net income			246,000,000				373,000,000
Comprehensive income (loss)			286,000,000				320,000,000
Former Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			818,000,000				1,508,000,000
Gas purchases			200,000,000				381,000,000
Operating expenses			125,000,000				234,000,000
General and administrative expenses			41,000,000				73,000,000
Depreciation, depletion and amortization			175,000,000				342,000,000
Taxes, other than income taxes			17,000,000				35,000,000
Total operating costs and expenses			558,000,000				1,065,000,000
Operating income (loss)			260,000,000				443,000,000
Gain (loss) on derivatives			93,000,000				64,000,000
Interest expense			7,000,000				15,000,000
Income (loss) before income taxes			346,000,000				492,000,000
Provision for income taxes			142,000,000				200,000,000
Net income			204,000,000				292,000,000
Comprehensive income (loss)			245,000,000				239,000,000
Other Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			127,000,000				248,000,000
Operating expenses			40,000,000				72,000,000
General and administrative expenses			7,000,000				12,000,000
Depreciation, depletion and amortization			12,000,000				24,000,000
Taxes, other than income taxes			3,000,000				6,000,000
Total operating costs and expenses			62,000,000				114,000,000
Operating income (loss)			65,000,000				134,000,000
Other gain, net			1,000,000
Interest expense			2,000,000				4,000,000
Income (loss) before income taxes			64,000,000				130,000,000
Provision for income taxes			22,000,000				49,000,000
Net income			42,000,000				81,000,000
Comprehensive income (loss)			$ 40,000,000				$ 79,000,000
7.35% Senior Notes due 2017 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.35%				7.35%
7.125% Senior Notes due 2017 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.125%				7.125%
7.15% Senior Notes due 2018 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.15%				7.15%
7.5% Senior Notes Due 2018 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	7.50%				7.50%
4.10% Senior Notes Due 2022 [Member]
Condensed Income Statements, Captions [Line Items]
Stated interest rate	4.10%				4.10%

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