Condensed Consolidating Financial Information (Condensed Consolidating Statement of Operations) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Condensed Income Statements, Captions [Line Items]
Operating revenues	$ 928.0		$ 868.0		$ 3,076.0		$ 2,464.0
Gas purchases - midstream services	220		195		752		575
Operating expenses	108		90		309		237
General and administrative expenses	54		51		162		136
Depreciation, depletion and amortization	238		205		693		571
Taxes, other than income taxes	22		18		72		59
Total Operating Costs and Expenses	642		559		1,988		1,578
Operating Income	286		309		1,088		886
Other Gain, Net					1
Gain (loss) on derivatives	78		12		(29)		75
Interest expense	13	[1]	11	[1]	39	[1]	29	[1]
Income Before Income Taxes	351		310		1,021		932
Provision (benefit) for income taxes	140	[1]	124	[1]	409	[1]	373	[1]
Net Income (Loss)	211		186		612		559
Comprehensive income	242		139		635		458
Eliminations [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			(86.0)				(246.0)
Operating expenses			(86)				(246)
Total Operating Costs and Expenses			(86)				(246)
Equity in earnings of subsidiaries			(186)				(559)
Income Before Income Taxes			(186)				(559)
Net Income (Loss)			(186)				(559)
Comprehensive income			(139)				(457)
Parent [Member]
Condensed Income Statements, Captions [Line Items]
Equity in earnings of subsidiaries			186				559
Income Before Income Taxes			186				559
Net Income (Loss)			186				559
Comprehensive income			139				458
Former Guarantors [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			823.0				2,331.0
Gas purchases - midstream services			195				575
Operating expenses			140				376
General and administrative expenses			44				117
Depreciation, depletion and amortization			192				534
Taxes, other than income taxes			16				50
Total Operating Costs and Expenses			587				1,652
Operating Income			236				679
Gain (loss) on derivatives			10				74
Interest expense			9				24
Income Before Income Taxes			237				729
Provision (benefit) for income taxes			91				291
Net Income (Loss)			146				438
Comprehensive income			98				337
Other Subsidiaries [Member]
Condensed Income Statements, Captions [Line Items]
Operating revenues			131.0				379.0
Operating expenses			36				107
General and administrative expenses			7				19
Depreciation, depletion and amortization			13				37
Taxes, other than income taxes			2				9
Total Operating Costs and Expenses			58				172
Operating Income			73				207
Gain (loss) on derivatives			2				1
Interest expense			2				5
Income Before Income Taxes			73				203
Provision (benefit) for income taxes			33				82
Net Income (Loss)			40				121
Comprehensive income			$ 41				$ 120
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.