For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
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-Section 50
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the equity method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value subject to the equity accounting method. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of each individual error being corrected in the correcting adjustment.

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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