OK, I have another way to explain what can be validated using XBRL calculations and what you need to use XBRL Formulas to validate.
Take a look at this PDF which has been annotated to show the validation of each computation in the XBRL instance.
Seems like a lot of work! And it is. Good thing that a computer application will do the vast majority of the validation for you. What is the point of all this validation. Well, remember the saying "garbage in, garbage out". That is why. If you want to automate processes, these numbers need to be squeaky clean.