Impact of Derivative Activities on Condensed Consolidated Statements of Income (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Commodity gains (losses) recognized in the Consolidated Statements of Operations	$ 88,422,000	$ (68,131,000)	$ 37,427,000
Cash Settlements Interest Rate Swap Operating Activities, Net	(2,210,000)	0	0
Cash Settlements Commodity Operating Activities	80,297,000	(4,136,000)	(48,291,000)
Commodity cash settlements and premiums received (paid) recognized in the Consolidated Statements of Cash Flows	78,087,000	(4,136,000)	(48,291,000)
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	(7,510,000)	0	0
Gain (Loss) on Commodity Derivative Instruments	$ (95,932,000)	$ 68,131,000	$ (37,427,000)