Discontinued Operations (Significant Operating and Investing Activities of Discontinued Operations) (Detail) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net Cash Provided by (Used in) Operating Activities, Discontinuing Operations [Abstract]
Asset impairments	$ 60,939	$ 108,184	$ 15,934
Dry hole and unproved leasehold impairment costs	0	67,639	29,528
Loss (gain) on sale of assets	13,664	706	(8,526)
Depreciation, depletion and amortization	11,447	45,085	41,715
Cash provided by (used in) operating activities, discontinued operations	16,478	202,073	65,295
Net Cash Provided by (Used in) Investing Activities, Discontinuing Operations [Abstract] [Abstract]
Oil and gas properties and equipment	(579)	(49,717)	(101,145)
Acquisition of oil and gas properties	0	0	(1,100)
Cash provided by (used in) investing activities, discontinued operations	76,526	(31,917)	(84,585)
Discontinued Operations
Net Cash Provided by (Used in) Operating Activities, Discontinuing Operations [Abstract]
Asset impairments	612	105,337	15,535
Dry hole and unproved leasehold impairment costs	0	67,639	29,528
Loss (gain) on sale of assets	13,664	706	(8,526)
Depreciation, depletion and amortization	2,202	28,391	28,758
Net Cash Provided by (Used in) Investing Activities, Discontinuing Operations [Abstract] [Abstract]
Oil and gas properties and equipment	(579)	(49,717)	(101,145)
Acquisition of oil and gas properties	0	0	(1,100)
Proceeds from sales of assets	$ 77,105	$ 17,800	$ 17,660