Revenue From Contracts with Customers - Disaggregate Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Disaggregation of Revenue
Net sales	$ 347,348	$ 286,987	$ 982,495	$ 886,351
Contract with Customer, Asset, Allowance for Credit Loss, Current					$ 1,500
Graphite Electrodes - Three-to-five-year take-or-pay contracts ("LTA")
Disaggregation of Revenue
Net sales	267,349	250,011	766,503	771,400
Graphite Electrodes - Short-term agreements and spot sales ("Non-LTA")
Disaggregation of Revenue
Net sales	69,295	32,303	181,754	93,232
By-products and other
Disaggregation of Revenue
Net sales	$ 10,704	$ 4,673	$ 34,238	$ 21,719