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Significant accounting policies - Intangible Assets (Details) (Capitalized computer software and development costs, Maximum) | 12 Months Ended
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Dec. 31, 2014
Capitalized computer software and development costs | Maximum |
Finite-Lived Intangible Assets [Line Items] |
Intangible assets, useful life | 4 years

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- Details
Name: | us-gaap_FiniteLivedIntangibleAssetsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
Name: | us-gaap_FiniteLivedIntangibleAssetUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration