Why would I use an extension?
XBRL stands for eXtensible Business Reporting Language. This means that it is a language for reporting business data that can be added to if necessary.
If you cannot find an appropriate tag for a line item you may want to consider using an extension to add a new tag to the taxonomy. This is one of the strengths of XBRL - it allows for extremely flexible financial reporting.
Custom line items
Let’s assume we ran an airline and had a line item for Flight Equipment. There is no tag in the core taxonomy that adequately describes Flight Equipment so this is a good reason to create an extension.
Some companies may choose to disaggregate concepts that the taxonomy reports together. For example, there is a tag called ifrs-full:SalesAndMarketingExpense which would be used if you reported all of your marketing expenses as a single line item. If, however, your report splits all your marketing expenses into separate line items (e.g. Radio advertising, TV advertising, Web advertising etc.), you would need to create extensions – one for each line – to successfully tag these cells. If we applied the ifrs-full:SalesAndMarketingExpense tag to all of these line items, we would create a Duplicate Mapping error.
What are anchors?
Although we can extend the taxonomy by adding new tags, these tags have to be connected to some pre-existing element in the taxonomy (i.e. the element we are extending).
Anchors should be an appropriate concept in the ESEF taxonomy which is either wider or narrower in accounting meaning or scope. Analysts, regulators and other users can use this information to better interpret and compare financial information collected through the use of extensions. In the flight equipment example, the nearest accounting concept is Property, Plant and Equipment. In our disaggregation example, the anchor would be the concept we are disaggregating (i.e. ifrs-full:SalesAndMarketingExpense).
You cannot create an extension without adding an anchor (with the one exception covered below). If you do not add an anchor you will get an Invalid anchoring validation error when you
validate the document. This is a critical error that will prevent you from correctly filing your report to your National Competent Authority (NCA).
How many anchors should I add?
You can apply as many anchors as needed to make your extensions clear and unambiguous. If you are adding more than one anchor you must specify which is the Wider Anchor - the tag with the widest possible applicability to the extension.
Custom extension properties
Because an extension is a custom tag for your unique report, you can set the fixed attributes that you can't change with a pre-existing tag. These include: Balance Type (i.e. whether this is Credit or Debit), the Item Type (e.g. Monetary, Share, Percentage etc.), the Period Type (i.e. Instant, Duration or Unknown), and the Label and Technical Name of your tag. Once you have created your extension, select the cell and click the Taxonomy Extension Properties tab to set these attributes.
If you are reporting in any language other than English, the ESMA manual recommends that reporters add a label in another language so that end-users can view your custom tags in their different languages (this is only a recommendation, not a requirement). This isn't necessary for ordinary XBRL taxonomy tags as they already have labels pre-written in other languages. The most relevant example would be that you were reporting in Swedish and wanted to create a label for your extension in English. Belgian reporters may also want to add additional labels for French and English if they are reporting in Dutch, for example. To do this, use the Label for additional cultures option in the Tagging Properties tab.
Extensions, anchors and subtotals
If you have subtotals in your table, but can’t find an appropriate pre-existing tag in the taxonomy, you will need to create an extension – but which pre-existing concept do you anchor it to? In this specific case, it is possible to create extensions for subtotals without anchoring them to anything; instead, you need to add a calculation to the subtotal cell. This will solve the critical ‘Invalid anchoring’ validation error which would otherwise appear.
If you do not have a row item label for the subtotal line, you will be prompted to create a name. You can always rename your extension by clicking on the Taxonomy Extension Properties tab and changing the Element Name.
I’m still not sure about whether I need to add an extension
We strongly recommend discussing with your auditors and doing some research if you are uncertain about whether you should use an extension for a specific line item or how you should anchor that extension. For more information on how to add extensions or dimension members to a table, please see our Tagger pages.