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Preferred share issuance and merger (Narrative) (Details) - USD ($) |  |  | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Aug. 11, 2015 | May 18, 2015 | Mar. 31, 2016 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | May 17, 2015
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Preferred stock, par value |  |  |  | $ 0.01 |  | $ 0.01 |  |
Max Reimbursement to Acquiror | $ 500,000 |  |  |  |  |  |  |
Early Repayment of Subordinated Debt |  |  |  | $ 304,782,000 | $ 0 |  |  |
Common stock, par value |  |  |  | $ 0.01 |  | $ 0.01 | $ 0.01 |
Goodwill, purchase accounting adjustments |  |  | $ 1,100,000 |  |  |  |  |
BCP IV GrafTech Holdings LP [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Amount |  | $ 854,337,000 |  |  |  |  |  |
Convertible Preferred Stock [Member] | BCP IV GrafTech Holdings LP [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of Shares |  | 150,000 |  |  |  |  |  |
Amount | $ 150,000,000 | $ 150,000,000 |  |  |  |  |  |
Convertible Preferred Stock [Member] | BCP IV GrafTech Holdings LP [Member] | Series A Preferred Stock [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of Shares | 136,616 |  |  |  |  |  |  |
Preferred stock, par value | $ 0.01 |  |  |  |  |  |  |
Business Combination, Equity Interest Transferred, Percentage of Common Stock Outstanding Before Transaction | 19.90% |  |  |  |  |  |  |
Convertible Preferred Stock [Member] | BCP IV GrafTech Holdings LP [Member] | Series B Preferred Stock [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of Shares | 13,384 |  |  |  |  |  |  |
Preferred stock, par value | $ 0.01 |  |  |  |  |  |  |
Business Combination, Equity Interest Transferred, Percentage of Common Stock Outstanding Before Transaction | 2.00% |  |  |  |  |  |  |
Preferred Stock [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Common stock, par value |  |  |  |  |  |  |  | $ 0.01
Convertible Preferred Stock, Conversion Price |  |  |  |  |  |  |  | $ 5.05
Senior Notes | Senior Subordinated Notes Due Nov 2015 [Member] |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Early Repayment of Subordinated Debt | $ 200,000,000 |  |  |  |  |  |  |
Notes Payable, Other Payables [Member] | Term Loan Facility |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Debt Instrument, Unused Borrowing Capacity, Amount | $ 40,000,000 |  |  |  |  |  |  |

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