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Fair value of financial and nonfinancial obligations.

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Fair Value Measurements - Balance Sheet Grouping (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial Assets
Cash equivalents	$ 32,774	$ 314,466	$ 275,841	$ 128,836
Deferred compensation plan	1,392	1,749
Level 1 [Member]
Financial Assets
Deferred compensation plan	1,392	1,749
Level 2 [Member]
Financial Assets
Deferred compensation plan	0	0
Level 3 [Member]
Financial Assets
Deferred compensation plan	0	0
Cash Equivalents - Money Market Funds [Member] \| Level 1 [Member]
Financial Assets
Cash equivalents	12,188	271,027
Cash Equivalents - Money Market Funds [Member] \| Level 2 [Member]
Financial Assets
Cash equivalents	0	0
Cash Equivalents - Money Market Funds [Member] \| Level 3 [Member]
Financial Assets
Cash equivalents	0	0
Commodity Derivatives [Member]
Financial Assets
Commodity derivatives	109,494	656
Financial Liabilities
Commodity derivatives	1,039	25,714
Commodity Derivatives [Member] \| Level 1 [Member]
Financial Assets
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	0	0
Commodity Derivatives [Member] \| Level 2 [Member]
Financial Assets
Commodity derivatives	109,494	656
Financial Liabilities
Commodity derivatives	1,039	25,714
Commodity Derivatives [Member] \| Level 3 [Member]
Financial Assets
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	0	0
Estimate of Fair Value Measurement [Member] \| Cash Equivalents - Money Market Funds [Member]
Financial Assets
Cash equivalents	$ 12,188	$ 271,027