XBRL-based structured digital financial reporting - Blog: Digital Financial Reporting (using XBRL)

I took the Comprehensive Example, created two additional copies (so that I have three companies), and now have the Comparison Example.

The Comparison Example is the capstone of what someone is likely trying to achieve in XBRL: comparison of some set of business information. There are three ways business information is commonly used:

Using a single information set: For example, using one financial report.
Comparing across periods: For example, comparing your financial report over a period of time.
Comparing across entities: For example, comparing three entities at one point in time or for a period of time.

The Comparison Example was created to test these use cases. These three XBRL instances and related XBRL taxonomies are as identical as they can be. Each company has its own meta data where it needs to.

When you look at this example, consider where the meta data is defined. There are four levels here in this example:

Company taxnoomy: Defining concets at this level provides the least comparability as it is defined at the company level, but at the same time it provides the most flexibility because the company can define anything it wants. If the GAAP taxonomy follows the FRLM and BRLM, then the compnay does also.
GAAP taxonomy: This gives improved comparability because different companies share the same GAAP. It does not have to be this way, conceivably every company could create their own XBRL taxonomy and create great XBRL. But, if every company did that individually, comparisons would still be possible, they would just be difficult becuase you would basically have to physically map one company to another.
FRLM (Financial Reporting Logical Model) taxonomy: This level provides comparability between GAAP taxonomies, should that be desired.
BRLM (Business Reporting Logical Model) taxonomy: This can be use by different domains who want to leverage the Business Reporting Logical Model. The advantage here is that if a software application were developed at this level, anything above it in this list would be comparable with that software. The software would still be XBRL compliant, i.e. the BRLM is 100% XBRL compliant.
XBRL: Defined by XBRL. Here, you have the least amount of flexibility (i.e. you must follow XBRL to be XBRL compliant) but you also have the most comparability. If everything was defined at this level it would be very comparable, but not very flexible. Basically, if everything were defined at this level it would serve one business domain and would basically be a form.

The level at which things are defined is determined by the business domain applying XBRL. The advantage of the BRLM level is that it allows users to be clear about the business semantics they are using and software applications can leverage that clarity, making the software and XBRL easier to work with.