The IFRS Foundation released business rules in the form of XBRL Formulas for the IFRS taxonomy. I would expect that the US GAAP Taxonomy will not be far behind.
I have been a long time proponent of XBRL Formula to express business rules which XBRL calculations cannot express. The fact is, it is extremely unlikely that you can create a quality XBRL instance without using XBRL Formula or something like it to be sure the relations between facts reported are correct.
For instance, the IFRS Foundation classifies their XBRL Formulas into the following categories (see page 2 of this document):
- Cross period validations – whereas formulae will ensure that calculation of roll-forwards from beginning balance over the total changes over the period equals ending balance.
- Earnings per share validations– whereas formulae will ensure that the EPS calculation of the Profit (loss) and the average number of shares provides correct results.
- Axis aggregation validations – whereas formulae will ensure that members of an axis are calculated to their parent members properly (if applicable for a given axis and only if the filer structured their members in a summation-like hierarchy).
- Fact equivalence validations– whereas formulae will ensure that if two facts are tagged by different IFRS Taxonomy concepts but, conceptually represent the same thing, that they are equal. Usually, one of them is in a dimensional context, whilst the other one is not (e.g. ‘Aircraft’ = ‘Aircraft [member]’ in ‘Property, plant and equipment [axis]’ with primary item ‘Property, plant and
- Common accounting equivalence validations – whereas formulae will ensure that general principles of accounting are applied correctly.
- Positive / negative fact validation – whereas formulae will ensure that if a fact is expected to be reported as an amount greater or equal zero it is not negative and vice versa.
- Percentage warnings– whereas formulae will ensure that the formal of percentage fact is appropriate according to the XBRL specification.
If you are not verifying these types of things to be sure they are correct, it is highly likely that they are not. Things may seem correct because you don't run the validation against your document. But if you do validate against business rules expressed using XBRL Formula, you might be surprised what you have missed.