This document serves to document the motivation behind certain design decisions made in the Open Information Model, including the constraints that limit certain XBRL v2.1 features documented in the xBRL-XML specification.
A key goal of the Open Information Model was to create a model that was syntax-independent, implying that there should be no dependency on XML or XML-related specifications.
The model does require a type system, and although XML Schema as a whole is very much XML-specific, the system that it defines for specifying simple datatypes is largely independent of XML. It specifies a lexical representation for each type that is a string that could be included in just about any other format.
The XML Schema datatype system (XSD) is very complete, and other, non-XML technologies have seen the value in reusing it. For example, the JSON-LD specification includes many examples that make use of XSD.
XBRL v2.1 permits an instance to reference its taxonomy through a combination
linkbaseRef elements. The XBRL instance schema requires
schemaRef elements to appear before
linkbaseRef elements. In certain
situations, the order of DTS discovery is significant, and as such, the order
of DTS references needs to be preserved.
In the long term, we expect taxonomy to move to non-XML-based representations, and it is likely that the URLs used for specifying DTS entry points will become pure identifiers. As such, the Working Group was keen to simplify the definition of the taxonomy entry point to a single list.
The ordering requirements of entry points consisting of a mix of
linkbaseRefs makes this inherently difficult and cumbersome, as it is
effectively two lists, and processors need to know whether each item is a
linkbase or a schema in order to preserve order correctly.
The Working Group could not find any examples of linkbases being referenced directly from an instance document, and as such, it was decided to apply the simplifying constraint than an instance's DTS may be specified only by a list of schemas.
Although fact identifiers do not contribute to the semantic meaning of a fact, they are potentially useful in providing the ability to trace an individual fact through different representations, for example, by allowing the easy tracing of a fact in an xBRL-JSON document back to its source in an Inline XBRL document.
The OIM provides separate definitions of "equality" and "equivalence" for both facts and reports, with facts that differ only in their identifier being considered equivalent but not equal.
Fact identifiers are mandatory in the model, and the xBRL-XML specification prescribes a standard approach for generating predictable IDs where they are not present in an xRBL-XML report.