Custom scoring using IDF of _all field?

We are querying 3 fields of a document using multi_match with best_fields. The 3 fields are also included in _all. I'm using a custom Similarity class which currently defaults the IDF value to 1. How can I use the true IDF value of the _all field as the IDF value of the remaining 3 fields?

maybe you can look this page,elasticsearch can controll TF/IDF open or close