Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets
Cash and cash equivalents	$ 169,593	$ 187,492
Short term investments	67,483	96,765
Restricted cash	18,962	5,953
Trade accounts receivable (net of $0 allowance at March 31, 2021)	129,086	110,869
Other receivables	3,764	3,053
Inventories	154,395	126,008
Other current assets	39,917	58,000
Total current assets	583,200	588,140
Property, plant and equipment
Property, plant and equipment, net	1,058,942	1,007,303
Other assets
Equity investments	74,503	71,783
Other noncurrent assets	57,513	55,246
Total other assets	132,016	127,029
Total assets	1,774,158	1,722,472
Current Liabilities
Accounts payable	122,916	103,743
Accrued expenses and other current liabilities	150,167	155,256
Current maturities of debt	24,597	31,097
Total current liabilities	297,680	290,096
Long-term debt	519,357	477,215
Asset retirement obligations	224,615	230,732
Accrued pension benefits	2,088	2,879
Accrued postretirement benefits other than pension	95,936	94,388
Accrued workers' compensation	249,133	244,695
Other noncurrent liabilities	103,906	98,906
Total liabilities	1,492,715	1,438,911
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,323 and 25,220 shares at March 31, 2021 and December 31, 2020, respectively	254	253
Paid-in capital	770,052	767,484
Retained earnings	372,882	378,906
Treasury stock, 10,088 shares at March 31, 2021 and December 31, 2020, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income (loss)	(34,364)	(35,701)
Total stockholders' equity	281,443	283,561
Total liabilities and stockholders' equity	$ 1,774,158	$ 1,722,472