From an XBRL perspective, since every concept in these equations is a credit including the total, the first equation is fine.
We cannot, however, rewrite the first equation as the second in XBRL because we cannot have a negative calculation weight on
the last two operands since those operands and the total are credits. Now given that the two calculations are logically
equivalent, it is sufficient to just include the first one in the XBRL for both. In fact, the first calculation is valid for
the cash flows statement even if that same data did not exist in the income statement.
One of the most common issues filers encounter deals with creating a calculation for a set of concepts for one table and
context and the calculation inadvertently applying to a very similar but different set of concepts for a different context.
Consider the following subsection of an income statement:
From this we have the following calculation:
Revenues - OperatingExpenses = OperatingIncomeLoss
Now let's consider a table in the notes that breaks down expenses based on industry segments via dimensions:
In this table, the Hotels and Corporate columns would be tagged with separate Business Segment dimensions, whereas the
Consolidated column would be dimensionless since it is the aggregate. The amounts 10,000 and 4,000 in the Consolidated
column would already have been tagged from the income statement.
The issue here arises when we now try to apply the calculation from the first table to the facts in the second table.
Remember when applying calculations, we do so by context which means the calculation from the first table gets applied
to each of the three columns of the second table. When we try to apply the calculation to the first two columns, there
is no "OperatingExpenses" fact for either of those contexts. This causes the calculation to be applied as follows:
Revenues = OperatingIncomeLoss
This is obviously wrong and will flag a calculation inconsistency for those first two columns or contexts. In these
circumstances, the SEC requires that the calculation for the income statement be included as it is valid when applied
to the "Required Context". The "Required Context" is the main dimensionless context of the document whose dates match
the reporting period of the filing.
One thing to remember is that the calculation will apply correctly to the Consolidated column. The facts for "Revenues"
and "OperatingIncomeLoss" in this column are the exact same facts as those in the income statement as they share the
same dimensionless context. This means the application of the calculation to the Consolidated column is one and the
same as the application of the calculation to the income statement.