In the actual calculation in the XBRL, the addition or subtraction of a concept is denoted by a calculation weight of either 
or [-1], respectively. Note, however, that this rule regarding balance types and valid calculations weights does not put any
restriction on whether the fact value itself is positive or negative. Rules regarding the sign of a fact value are inherent to
the concept definition and are not determined by the balance type. For example, "ProfitLoss" can be either positive or negative,
but "CommonStockSharesOutstanding" cannot be negative. This is important to remember as one could incorrectly adjust the sign of
a fact value to overcome calculation weight restrictions based on balance type. Such action makes the calculation "work", but
renders the data meaningless. We will show an example of this later.
The following are valid calculations based on balance type:
The following are invalid calculations based on balance type:
Finally, a concept's period type plays a role in how we can set up calculations. A single calculation can only contain concepts of
the same period type. This means instants can only add up to instants and durations can only add up to durations.