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Fair value of financial and nonfinancial obligations.

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Fair Value Measurements - Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Cash equivalents	$ 16,449	$ 32,774	$ 314,466	$ 275,841
Deferred compensation plan	2,033	1,392
Level 1
Financial Assets
Deferred compensation plan	2,033	1,392
Level 2
Financial Assets
Deferred compensation plan	0	0
Level 3
Financial Assets
Deferred compensation plan	0	0
Money Market Funds \| Level 1
Financial Assets
Cash equivalents	0	12,188
Money Market Funds \| Level 2
Financial Assets
Cash equivalents	0	0
Money Market Funds \| Level 3
Financial Assets
Cash equivalents	0	0
Commodity Derivatives
Financial Assets
Commodity derivatives	8,890	109,494
Financial Liabilities
Commodity derivatives	10,056	1,039
Commodity Derivatives \| Level 1
Financial Assets
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	0	0
Commodity Derivatives \| Level 2
Financial Assets
Commodity derivatives	8,890	109,494
Financial Liabilities
Commodity derivatives	10,056	1,039
Commodity Derivatives \| Level 3
Financial Assets
Commodity derivatives	0	0
Financial Liabilities
Commodity derivatives	0	0
Estimate of Fair Value Measurement \| Money Market Funds
Financial Assets
Cash equivalents	$ 0	$ 12,188