XBRL International provided a document, Technical Considerations for Use of XBRL Dimensions 1.0, which clarifies semantics carried by information expressed using XBRL Dimensions. The document is worth reading but here are the highlights:
- <segment> and <scenario> elements carry no meaning: Where you put XBRL Dimensions information has no semantics. Whether you use the <segment> element of the context or the <scenario> element; the meaning is the same. XBRL International recommends using either <segment> or <scenario>; for new taxonomies they recommend using <scenario>.
- Always close hypercubes: All hypercubes (positive) should be closed. Don't use negative hypercubes without an associated positive hypercube.
- Don't mix dimensional and nondimensional models: The statement (Recommendation 3.5), "Where a taxonomy makes use of dimensions, all concepts should be associated with at least one hypercube, even if that hypercube has no associated dimensions." basically means don't mix a dimensional model and a nondimensional model.
- Dimension defaults are used to represent a total: The statement (Recommendation 5.1) "Dimension defaults should only be used to specify a member which is the natural total for a domain." basically means that dimension defaults convey information about a total.
- Don't use complex typed dimensions: The statement (Recommendation 6.1) "Typed dimensions should only use XML Schema simple types as their data type." basically means don't use complex typed dimensions, use only simple typed dimensions only.
The US GAAP XBRL Taxonomy currently violates two of these recommendations.
The US GAAP XBRL Taxonomy mixes dimensional and nondimensional models. Not every concept belongs to a [Table] (which is the US GAAP XBRL Taxonomy name for hypercube). But that does not mean that public companies creating XBRL-based financial filings cannot conform to this XBRL International recommendation. In fact, some public companies already do this. Come public companies always represent their information associating all reported facts to a [Table].
The US GAAP XBRL Taxonomy also does not use dimension defaults per the XBRL International recommendations. Many times dimension defaults are used to convey information which is not a total. Also, many people think that dimensions defaults are about "defaults". They are not. They are about intersections between two or more different [Table]s or hypercubes. Generally one hypercube is summary and the other hypercube is a breakdown of that total.