Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Asset (liability)
Post-employment benefits other than pensions	$ 1,714	$ 1,564
Other reserves	1,901	226
Deferred compensation	3,263	3,322
Environmental reserves	1,358	1,301
Revenue recognition	5,027	0
Lease liabilities	11,639	10,469
Interest expense carryforward	14,163	0
Pensions	1,393	7,456
Accrued compensation expense	6,410	2,683
Net operating loss and credit carryforwards	11,423	12,711
Subtotal	58,291	39,732
Valuation allowance	(4,957)	(14,134)
Total deferred tax assets	53,334	25,598
Depreciation	(24,484)	(12,112)
Lease assets	(11,184)	(10,261)
Inventory	(2,329)	(3,532)
Amortization	(35,542)	(10,754)
Mine development	(917)	(1,669)
Unrealized gains	(663)	0
Total deferred tax liabilities	(75,119)	(38,328)
Net deferred tax liabilities	$ (21,785)	$ (12,730)