Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative [Line Items]
Unrealized pre-tax derivative loss recorded in AOCI			$ 358,000
Unrealized after tax derivative loss recorded in AOCI			(222,000)
Gain (Loss) Recognized in Income (Ineffective Portion)	0	(39,000)	0	(2,930,000)
Unrealized net gains, reclassify into earnings				3,900,000
Unrealized pre-tax derivative loss expected to be reclassified into earnings			(358,000)
Basis Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	(1,500,000)		(1,500,000)
Volume Hedged	70,593		70,593
Designated as Hedging Instrument
Derivative [Line Items]
Gains (Losses) in oil and gas sales related to settled hedging transactions	3,500,000	27,400,000	10,600,000	94,400,000
Not Designated as Hedging Instrument
Derivative [Line Items]
Accumulated other comprehensive income, unrealized net gains			103,600,000
Accumulated other comprehensive income, unrealized net gains, after tax			63,200,000
Derivatives Excluding Basis Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 63,300,000		$ 63,300,000