Stock Based Compensation (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Class of Stock [Line Items]
Natural gas and oil properties	$ 18	$ 30	$ 55	$ 90
General and administrative expenses	17	24	55	71
Natural gas, oil and NGL production expenses	6	8	18	26
Marketing, gathering and compression expenses	4	5	12	14
Oilfield services expenses	2	3	8	8
Total	$ 47	$ 70	$ 148	$ 209
Vesting period for non employee directors years			3 years
Vesting period for employees years			4 years
Restricted stock
Class of Stock [Line Items]
Vesting period for non employee directors years			2 years 7 months 6 days