XBRL (eXtensible Business Reporting Language) is the standard companies use to tag each figure in their financial statements with a defined meaning: what it is (the concept), the period it covers, its unit and its scale. A tagged figure can be read by software without guessing.
How it is filed
Since 2019 to 2021, depending on company size, the tags are embedded in the HTML of the filing itself (inline XBRL), so the readable document and the data are the same file. Financial data in XBRL starts with periods ending after June 2009 for the largest companies and June 2011 for most others.
Concepts and taxonomies
Concepts come from published taxonomies: us-gaap for US accounting rules, ifrs-full for IFRS, and dei for cover page facts such as the company's name and share count. A company that believes no standard concept fits can create its own, called an extension.
On Listed
The SEC publishes every company's tagged facts as "company facts" data. Listed maps them to about 45 standard line items, each with an ordered list of standard concepts to try. A figure tagged with an extension falls outside the map and is shown as not reported.
Each figure's note gives its concept, its period, the form and the accession number of the filing it came from.