New Accounting Standards - Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Income [Line Items]
AOCI at beginning of period, gross	$ 137,555	$ 254,678
Derivative contract settlements reclassified to natural gas, NGLs and oil sales, gross	(24,328)	(57,629)
Amortization related to de-designated hedges, gross	(12,172)
Change in unrealized deferred hedging gains, gross	(9,169)	131,081
AOCI at March 31, gross	91,886	328,130
AOCI at beginning of period, tax effect	(53,646)	(98,051)
Derivative contract settlements reclassified to natural gas, NGLs and oil sales, tax effect	9,488	22,187
Amortization related to de-designated hedges, tax effect	4,747
Change in unrealized deferred hedging gains, tax	3,576	(52,107)
AOCI at March 31, tax effect	(35,835)	(127,971)
AOCI at beginning of period, net of tax	83,909	156,627
Derivative contract settlements reclassified to natural gas, NGLs and oil sales, net of tax	(14,840)	(35,442)
Amortization related to de-designated hedges, net of tax	(7,425)
Change in unrealized deferred hedging gains, net of tax	(5,593)	78,974
AOCI at March 31, net of tax	$ 56,051	$ 200,159