Idf values of English words
I don't believe that there are any precalculated idf values out there. Inverse Document Frequency (idf) is the inverse of the number of documents in which a particular word appears in your corpus. If you only have one document, I'm afraid that value is simply 1. 

However, if you are looking to get rid of words such as the, as, and it which don't carry much meaning, nltk in Python has some useful tools to remove these "stop words" from your document and might help you. 

Here is a helpful example.