Revenue From Contracts with Customers - Performance Obligation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total Revenues	$ 347,348	$ 286,987	$ 982,495	$ 886,351
Graphite Electrodes - Three-to-five-year take-or-pay contracts ("LTA")
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total Revenues	267,349	$ 250,011	766,503	$ 771,400
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 233,000		$ 233,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 months		3 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 910,000		$ 910,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year		1 year
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 350,000		$ 350,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	2 years		2 years
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 278,000		$ 278,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	3 months		3 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 1,010,000		$ 1,010,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	1 year		1 year
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 450,000		$ 450,000
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Period	2 years		2 years