Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets
Cash and cash equivalents	$ 105,157	$ 153,020
Short term investments	129,057	135,667
Trade accounts receivable (net of $316 allowance at March 31, 2020)	149,225	168,125
Other receivables	19,016	21,143
Inventories	149,986	130,898
Other current assets	87,096	97,894
Total current assets	639,537	706,747
Property, plant and equipment, net	1,035,776	984,509
Other assets
Equity investments	106,535	105,588
Other noncurrent assets	75,658	70,912
Total other assets	182,193	176,500
Total assets	1,857,506	1,867,756
Current Liabilities
Accounts payable	108,972	133,060
Accrued expenses and other current liabilities	143,054	157,167
Current maturities of debt	28,456	20,753
Total current liabilities	280,482	310,980
Long-term debt	330,036	290,066
Asset retirement obligations	239,969	242,432
Accrued pension benefits	18,897	5,476
Accrued postretirement benefits other than pension	80,069	80,567
Accrued workers' compensation	218,626	215,599
Other noncurrent liabilities	95,663	82,100
Total liabilities	1,263,742	1,227,220
Stockholders' equity
Common stock, $0.01 par value, authorized 300,000 shares, issued 25,231 and 25,220 shares at March 31, 2020 and December 31, 2019, respectively	252	252
Paid-in capital	734,315	730,551
Retained earnings	698,292	731,425
Treasury stock, 10,088 shares at March 31, 2020 and December 31, 2019, respectively, at cost	(827,381)	(827,381)
Accumulated other comprehensive income (loss)	(11,714)	5,689
Total stockholders' equity	593,764	640,536
Total liabilities and stockholders' equity	$ 1,857,506	$ 1,867,756