Revenue From Contracts with Customers - Disaggregate Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenues	$ 451,899
Graphite Electrodes - Three-to-five-year contracts
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenues	272,201
Graphite Electrodes - Short-term contracts
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenues	167,595
By products
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenues	$ 12,103