Derivatives - Fair value of derivative instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Interest rate swap contract \| Derivative instruments designated as cash flow hedges:
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives	$ 561	$ 372
Interest rate swap contract \| Derivative instruments designated as cash flow hedges: \| Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net	300
Fair value of asset derivatives	332	134
Interest rate swap contract \| Derivative instruments designated as cash flow hedges: \| Other assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives	229	238
Foreign currency forward contracts \| Derivative instruments not designated as cash flow hedges:
Derivatives, Fair Value [Line Items]
Derivative assets (liabilities), at fair value, net	667	390
Foreign currency forward contracts \| Derivative instruments not designated as cash flow hedges: \| Prepaids and other current assets
Derivatives, Fair Value [Line Items]
Fair value of asset derivatives	0	390
Foreign currency forward contracts \| Derivative instruments not designated as cash flow hedges: \| Accrued liabilities
Derivatives, Fair Value [Line Items]
Fair value of liability derivatives	$ 667	$ 0

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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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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- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-SubTopic 30
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