Perhaps XBRL’s most-valuable feature is its ability to explain why an exceptional case exists in a report. A reporting company can address exceptions within the data through notes and tags, and avoid having a report fail the validation process. These notes are useful if there is a reasonable explanation for missing elements in the report, for example.
Although it’ll take a much deeper dive into XBRL to confirm, I expect that these notes and tags are built into the XBRL schema, and appear within XBRL documents. These notes and tags are also examples of embedded annotations. External annotations, based on XPointer, might also be interesting to consider in this context.