How to show extreme gratitude in an email? When contacting universities, in many cases, I find the replies to my emails extremely well detailed and helpful, therefore in such cases I find a simple "Thank you" to be just insufficient.

For example, when you have already applied to a program, and then you email the department to ask a simple question about supplementary documents, but they put SO MUCH effort in their response and even analyze your application before the application process has even started and they tell you that they are very certain that you will be accepted. How would you react? Is it rude to say something like: "I'm very glad to hear that."

In such situations, I do not really know how to best answer the person to show them that I really appreciate their email very much.
Politeness never hurts. "Thank you for your very detailed and positive information" could be one option or something along those lines. I am sure most people providing this type of response do not expect much in return (not that they do not deserve it). But as with many other instances, keeping the thanks short and concise is necessary, do not overwork it because that just seems suspicious. Remember that the person is doing their work (well) and should be praised for just that. 

If the school to which you are applying is large then it is not very likely you will be remembered and so the response will have little significance other than to show appreciation. In a smaller school, however, your politeness may be noticed and can help you build good relations with administration for the future so politeness never hurts.