Discontinued Operations (Significant Operating and Investing Activities of Discontinued Operations) (Detail) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Cash Provided by (Used in) Operating Activities, Discontinuing Operations [Abstract]
Asset impairments	$ 47,172	$ 60,939	$ 108,184
Dry hole and unproved leasehold impairment costs	0	0	67,639
Loss (gain) on sale of assets	197	13,664	706
Depreciation, depletion and amortization	5,463	11,447	45,085
Cash provided by (used in) operating activities, discontinued operations	(2,803)	16,478	202,073
Net Cash Provided by (Used in) Investing Activities, Discontinuing Operations [Abstract] [Abstract]
Oil and gas properties and equipment	(2,400)	(579)	(49,717)
Cash provided by (used in) investing activities, discontinued operations	200	76,526	(31,917)
Discontinued Operations
Net Cash Provided by (Used in) Operating Activities, Discontinuing Operations [Abstract]
Asset impairments	0	612	105,337
Changes in accounts payable and other accrued liabilities	(3,000)	0	0
Dry hole and unproved leasehold impairment costs	0	0	67,639
Loss (gain) on sale of assets	197	13,664	706
Depreciation, depletion and amortization	0	2,202	28,391
Net Cash Provided by (Used in) Investing Activities, Discontinuing Operations [Abstract] [Abstract]
Oil and gas properties and equipment	0	(579)	(49,717)
Proceeds from sales of assets	$ 200	$ 77,105	$ 17,800