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Significant accounting policies - Revenue Recognition (Details) | 12 Months Ended
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Dec. 31, 2015
Minimum |
Property, Plant and Equipment [Line Items] |
Term of contract (less than one year) | 1 year

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Period Type: | duration

X
- Definition 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. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |