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Preferred share issuance and merger (Net identifiable assets acquired) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 18, 2015
Business Acquisition [Line Items]            
Goodwill $ 171,117 $ 171,117 $ 171,117 $ 171,117 $ 172,059  
BCP IV GrafTech Holdings LP [Member]            
Business Acquisition [Line Items]            
Cash           $ 25,032
Accounts receivable           94,298
Inventories           344,765
Property, plant and equipment           650,405
Intangible assets           155,700
Deferred tax assets           41,606
Prepaid and other current assets           49,716
Other non-current assets           8,428
Accounts payable           (68,005)
Short-term debt           (18,779)
Other accrued liabilities           (53,252)
Long-term debt           (367,811)
Other long-term liabilities           (101,648)
Deferred tax liabilities           (79,235)
Net identifiable assets acquired           681,220
Goodwill           173,117
Net assets acquired           $ 854,337