Difference between persist () and save () hibernate?

I'm wanting to save an External User this is my controller that interacts with the BD

The thing that when I want to save a UsuarioExterno with all your documents I see this error.

  

org.hibernate.PersistentObjectException: detached entity passed to   persist

I was researching it and I realized that in my constructor I am inserting an Object "tipo_documento" that already exists in bd and does not create it again because it is already created, and save only saves me The father of the objects. How could I solve it?