You need to find out if a Coggin's certificate is required. If so this requires a fairly recent blood test so it's best to allow two weeks at least for results to return. It could be longer. I hope someone can give you more specifics.
Idaho is a brand inspection state. If you don't have a brand inspection from the state you are in, make sure you have some type of "proof of ownership" and contact the Idaho Brand Inspector office prior to traveling into Idaho.
As already mentioned, you will usually need a Coggins and health certificate when crossing state lines. The brand inspector's office could also clarify those details for you.