Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Derivative [Line Items]
Unrealized pre-tax derivative gain recorded in AOCI			$ 3,100,000
Unrealized after tax derivative gain recorded in AOCI			1,900,000
Gain (Loss) Recognized in Income (Ineffective Portion)	0		0	(2,891,000)
Unrealized net gains, reclassify into earnings				3,200,000
Unrealized pre-tax derivative gain expected to be reclassified into earnings			3,100,000
Designated as Hedging Instrument
Derivative [Line Items]
Gains (Losses) in oil and gas sales related to settled hedging transactions	4,900,000	30,500,000	7,100,000	67,000,000
Not Designated as Hedging Instrument
Derivative [Line Items]
Unrealized pre-tax derivative gain recorded in AOCI			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	68,500,000		68,500,000
Basis Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 8,700,000		$ 8,700,000
Volume Hedged	215,693		215,693