CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 221,947	$ 211,916
Short-term investments	8,504	8,504
Trade accounts receivable	76,120	83,298
Inventories, net	135,696	118,713
Prepaid expenses and other receivables	34,859	45,052
Total current assets	477,126	467,483
Mineral interests, net	98,395	100,855
Property, plant and equipment, net	640,732	637,108
Deferred income taxes	150,741	174,372
Other long-term assets	12,997	14,118
Total assets	1,379,991	1,393,936
Current liabilities:
Accounts payable	65,336	59,110
Accrued expenses	62,901	86,108
Short term financing lease liabilities	23,619	14,385
Other current liabilities	9,107	10,715
Total current liabilities	160,963	170,318
Long-term debt	380,098	379,908
Asset retirement obligations	60,746	57,553
Long term financing lease liabilities	40,662	24,091
Other long-term liabilities	37,219	36,825
Total liabilities	679,688	668,695
Stockholders’ Equity:
Common stock, $0.01 par value per share (Authorized -140,000,000 shares as of March 31, 2021 and December 31, 2020, 53,567,566 issued and 51,345,725 outstanding as of March 31, 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 March 31, 2021 and December 31, 2020)	(50,576)	(50,576)
Additional paid in capital	248,774	249,746
Retained earnings	501,569	525,537
Total stockholders’ equity	700,303	725,241
Total liabilities and stockholders’ equity	$ 1,379,991	$ 1,393,936