Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives	$ (6,891)	$ 24,248	$ 109,879	$ (30,166)
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	32	0	(7,527)	0
Gain (Loss) on Commodity Derivative Instruments	(6,923)	24,248	117,406	(30,166)
Cash Settlements Interest Rate Swap Operating Activities	(919)	0	(1,287)	0
Cash Settlements Commodity Operating Activities	7,337	(423)	66,582	(4,330)
Cash settlements and premiums received (paid), net	$ 6,418	$ (423)	$ 65,295	$ (4,330)