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Fair Value Measurements - Fair Value, Balance Sheet Grouping (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
Financial Assets
Cash equivalents	$ 16,112	$ 32,774	$ 107,379	$ 314,466
Commodity derivatives	19,226	108,455
Financial Liabilities
Commodity derivatives	75	0
Total
Financial Assets
Cash equivalents	4,344	12,188
Deferred compensation plan	1,816	1,392
Commodity derivatives	26,387	109,494
Financial Liabilities
Commodity derivatives	7,236	1,039
Level 1
Financial Assets
Cash equivalents	4,344	12,188
Deferred compensation plan	1,816	1,392
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	0	0
Level 2
Financial Assets
Cash equivalents	0	0
Deferred compensation plan	0	0
Commodity derivatives	26,387	109,494
Financial Liabilities
Commodity derivatives	7,236	1,039
Level 3
Financial Assets
Cash equivalents	0	0
Deferred compensation plan	0	0
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	$ 0	$ 0