Derivative Activities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Institution Counterparty	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Gain (Loss) Recognized in Income (Ineffective Portion)	$ 0	$ (2,930,000)	$ 1,125,000
Number of financial institutions with whom we conduct derivative activities	15
Number of secured lenders not in banking credit facility	1
Basis Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	1,700,000
Volume Hedged	35,164
Designated as Hedging Instrument
Derivative [Line Items]
Gains (Losses) in oil and gas sales related to settled hedging transactions	10,200,000	116,500,000	236,300,000
Not Designated as Hedging Instrument
Derivative [Line Items]
Accumulated other comprehensive income, net gains	103,600,000
Accumulated other comprehensive income, net gains, after tax	63,200,000
Derivatives Excluding Basis Swaps
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 401,700,000