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To identify and correct an outline structure, first understand what a correct outline structure looks like.
Basic XBRL outline structure
Note: The basic rules for the XBRL outline are the same for any taxonomy you use (US GAAP or IFRS) for the SEC.
In a basic XBRL outline, each section corresponds to a statement, note, or schedule within the quarterly or annual report. Within the sections, subsections relate to different levels of tagging requirements. The four levels of tagging requirements within Notes to the Financial Statements are:
Level 1 for text block
Level 2 for policy text block
Level 3 for table text block
Level 4 for detail tagging
Each subsection contains the concepts applied to the document and additional elements that are required for organization.
The following image shows an example of a basic outline structure within a subsection. It contains a Root abstract for organizational purposes and Line item concepts that are applied in the document.
When adding and organizing concepts within the XBRL outline, make sure the structure mirrors the presentation of the document as closely as possible. Sometimes it may not match perfectly due to rules on how certain elements should be organized within the outline. Ignoring these rules results in validation issues because the outline structure is not technically compliant with the rules set by the SEC.
Dimensional outline structure
When an axis and a member are applied to a fact it is referred to as a dimension. The dimensional structure has its own pattern of organizing items. If this structure is not set up correctly, it can cause XBRL validation errors.
Whenever you apply a member to a fact, make sure you also have a related dimensional structure in the outline.
The following image shows a correct dimensional structure with a hypercube (also known as a table) nested under the root abstract in the subsection. We use tables to organize dimensional items in the outline. In this situation, the table contains the axis(es), domain, member(s), line item abstract, and any concepts that are related.
Resolving outline structure issues
Below are the four most common outline issues and how to resolve them:
Hypercube is missing an axis
In order to correct this issue, add appropriate axis (more than one, if necessary), domain, and members to the outline as a child of the table.
An axis is missing a domain
In order to correct this issue, add a domain to the outline. Make sure all appropriate members are children of the domain.
An incorrect item is a child of an axis
In order to correct this issue, promote all children from underneath the member you want to remove. Remove the incorrect member. Add a domain as a child to the table. Demote all of the remaining members underneath the domain.
Line item concepts are not nested underneath the line items abstract, but have members applied
Line items cannot inherit any members that are applied to them, unless they are children of the member in the outline structure. In order to correct this issue, demote line item concepts to be children of the line items abstract.