CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 268,395	$ 211,916
Short-term investments	8,505	8,504
Trade accounts receivable	73,270	83,298
Inventories, net	94,026	118,713
Prepaid expenses and other receivables	35,034	45,052
Total current assets	479,230	467,483
Mineral interests, net	94,894	100,855
Property, plant and equipment, net	606,750	637,108
Deferred income taxes	151,934	174,372
Other long-term assets	11,799	14,118
Total assets	1,344,607	1,393,936
Current liabilities:
Accounts payable	39,138	59,110
Accrued expenses	58,114	86,108
Short term financing lease liabilities	22,421	14,385
Other current liabilities	14,721	10,715
Total current liabilities	134,394	170,318
Long-term debt	340,491	379,908
Asset retirement obligations	62,340	57,553
Long term financing lease liabilities	34,805	24,091
Other long-term liabilities	37,214	36,825
Total liabilities	609,244	668,695
Stockholders’ Equity:
Common stock, $0.01 par value per share (Authorized -140,000,000 shares as of September 30, 2021 and December 31, 2020, 53,632,812 issued and 51,410,971 outstanding as of September 30, 2021 and 53,408,040 issued and 51,186,199 outstanding as of December 31, 2020)	536	534
Preferred stock, $0.01 par value per share (10,000,000 shares authorized, no shares issued and outstanding)	0	0
Treasury stock, at cost (2,221,841 shares as of September 30, 2021 and December 31, 2020)	(50,576)	(50,576)
Additional paid in capital	255,317	249,746
Retained earnings	530,086	525,537
Total stockholders’ equity	735,363	725,241
Total liabilities and stockholders’ equity	$ 1,344,607	$ 1,393,936