Other non-current liabilities - Summary of Other Non-Current Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Liabilities, Noncurrent [Abstract]
Derivative instruments	$ 5,788	$ 322
Unrecognized tax benefits	892	892
Deferred rent	7,632	8,176
Retirement benefits	3,363	3,377
Other non-current liabilities	1,155	3,435
Non-current liabilities	$ 18,830	$ 16,202

For a classified balance sheet, the cumulative difference between the rental income or payments required by a lease agreement and the rental income or expense recognized on a straight-line basis, or other systematic and rational basis more representative of the time pattern in which use or benefit is granted or derived from the leased property, expected to be recognized in income or expense, by the lessor or lessee, respectively, more than one year after the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.26(c))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Section 25
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Fair value of liability arising from contracts that meet the criteria of being accounted for as derivative instruments, financial instrument or contract used to mitigate a specified risk (hedge), energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps.

+ References

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Aggregated carrying amounts of obligations as of the balance sheet date, excluding long-term debt, incurred as part of the normal operations that are expected to be paid after one year or beyond the normal operating cycle, if longer. Alternate captions include Total Deferred Credits and Other Liabilities.

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Amount recognized for uncertainty in income taxes classified as noncurrent.

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Amount of liabilities classified as other, due after one year or the normal operating cycle, if longer.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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