Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Derivative [Line Items]
Unrealized pre-tax derivative gain recorded in AOCI	$ 8,100,000
Unrealized pre-tax derivative gain recorded in AOCI	5,000,000
Gain (Loss) Recognized in Income (Ineffective Portion)	0	(2,891,000)
Unrealized net gains, reclassify into earnings		2,300,000
Unrealized pre-tax derivative gain expected to be reclassified into earnings	8,100,000
Designated as Hedging Instrument
Derivative [Line Items]
Gains (Losses) in oil and gas sales related to settled hedging transactions	2,200,000	36,500,000
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	53,400,000
Basis Swap
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 3,700,000
Volume Hedged	254,164