Derivative Instruments - Natural Gas Production/Processing Derivative Activities Table (Detail) (Price Risk Derivative [Member], Natural Gas [Member], Not Designated as Hedging Instrument [Member])	9 Months Ended
	Sep. 30, 2013
April 2013 to October 2013 Settlement [Member] | Two Way Collars [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	600,000	[1]
April 2013 to October 2013 Settlement [Member] | Two Way Collars [Member] | Call Option [Member] | Short [Member]
Average price per MMBtu
Average ceiling price	4.00
April 2013 to October 2013 Settlement [Member] | Two Way Collars [Member] | Put Option [Member] | Long [Member]
Average price per MMBtu
Average floor price	3.18
2013 Settlement [Member] | Three-Way Collars [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	0	[2]
2013 Settlement [Member] | Three-Way Collars [Member] | Call Option [Member] | Short [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	450,000
2013 Settlement [Member] | Three-Way Collars [Member] | Call Option [Member] | Long [Member] | Offsetting Purchased and Sold Positions For Four Hundred Fifty Thousand Million British Thermal Units Per Day [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	(450,000)
2013 Settlement [Member] | Fixed-Price Contracts [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	1,185,000
Average price per MMBtu
Average price per MMBtu	4.00
2014 Settlement [Member] | Three-Way Collars [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	600,000
2014 Settlement [Member] | Three-Way Collars [Member] | Call Option [Member] | Short [Member]
Average price per MMBtu
Average ceiling price	5.01
2014 Settlement [Member] | Three-Way Collars [Member] | Put Option [Member] | Short [Member]
Average price per MMBtu
Average floor price	2.75
2014 Settlement [Member] | Three-Way Collars [Member] | Put Option [Member] | Long [Member]
Average price per MMBtu
Average floor price	3.75
2014 Settlement [Member] | Fixed-Price Contracts [Member]
Derivative [Line Items]
Derivative nonmonetary notional amount per day	600,000
Average price per MMBtu
Average price per MMBtu	4.26

[1] The two-way collars have a contract term of April 2013 to October 2013.

[2] The Company entered into offsetting purchased and sold natural-gas three-way collars of 450,000 MMBtu/d for 2013 settlement.