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Discontinued Operations (Summarized Results from Discontinued Operations) (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Discontinued Operations and Disposal Groups [Abstract]    
Revenues $ 0 $ 9
Cost of oil and gas producing activities 0 (6)
Other operating expenses 0 (54)
Loss from discontinued operations before income taxes 0 (51)
Income tax benefit 0 469
Income from discontinued operations, net of taxes $ 0 $ 418