Discontinued Operations (Summarized Results from Discontinued Operations) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
Revenues									$ 15	$ 5,862	$ 43,845
Cost of oil and gas producing activities									(52)	(6,578)	(221,402)
Other operating expenses									226	(7,754)	(10,363)
Income (loss) from discontinued operations before income taxes									189	(8,470)	(187,920)
Gain (loss) on sale of assets before income taxes									(197)	(13,664)	(706)
Income tax benefit									46,039	5,269	2,496
Net income (loss) from discontinued operations available for diluted earnings per share	$ 7,191	$ 37,193	$ 1,229	$ 418	$ 563	$ (7,164)	$ (2,048)	$ (8,216)	$ 46,031	$ (16,865)	$ (186,130)