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Summary of significant accounting policies (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2017
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership percentage |  | 100.00% |  |  |  |
Allowance for doubtful accounts receivable | $ 1,000 | $ 1,000 | $ 1,000 |  |  |
BHH impairment |  | $ 0 | 11,584 | $ 0 |  |
Minimum [Member] | Building and Improvements [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 10 years |  |  |  |
Minimum [Member] | Machinery and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 5 years |  |  |  |
Minimum [Member] | Technology and Software [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 3 years |  |  |  |
Maximum [Member] | Building and Improvements [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 45 years |  |  |  |
Maximum [Member] | Machinery and Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 35 years |  |  |  |
Maximum [Member] | Technology and Software [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Useful life |  | 7 years |  |  |  |
Baise Haohi Carbon Co Ltd [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership percentage, equity method |  | 40.00% |  |  |  |
BHH [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership percentage, equity method |  | 40.00% |  |  |  | 40.00%
BHH impairment | $ 11,584 |  |  |  |  |
Retained Earnings [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Adjustment to beginning retained earnings as of December 31, 2013 |  |  |  |  | $ 9,059 |
Glencore [Member] |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Ownership percentage |  | 47.50% |  |  |  | 47.50%
Deferred Tax Liability [Member] |  |  |  |  |  |
Property, Plant and Equipment [Line Items] |  |  |  |  |  |
Adjustment to beginning retained earnings as of December 31, 2013 |  |  | $ 9,059 |  |  |

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