Derivative Activities - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2013 2013	Dec. 31, 2014 2013	Sep. 30, 2013 Designated as Hedging Instrument	Sep. 30, 2012 Designated as Hedging Instrument	Sep. 30, 2013 Designated as Hedging Instrument	Sep. 30, 2012 Designated as Hedging Instrument
Derivative [Line Items]
Derivative assets liabilities at fair value net	$ 66,000,000		$ 66,000,000
Unrealized pre-tax derivative gain recorded in AOCI			32,300,000
Unrealized after tax derivative gain recorded in AOCI			19,700,000
Gains (Losses) in oil and gas sales related to settled hedging transactions							27,400,000	61,500,000	94,400,000	197,700,000
Ineffective unrealized and realized gains (losses) recognized income	39,000	3,719,000	2,930,000	1,610,000
Unrealized net gains, reclassify into earnings			3,900,000
Accumulated other comprehensive income, unrealized net gains			103,600,000
Accumulated other comprehensive income, unrealized net gains, after tax			63,200,000
Unrealized pre-tax derivative gain expected to be reclassified into earnings					$ 22,100,000	$ 10,200,000