How can I hide the entity ID on an autocomplete widget?
You should have a look on Chosen module. 


  Chosen uses the Chosen jQuery plugin to make your  elements
  more user-friendly.


Follow this steps:


Install, enable and configure the Chosen module.
Change the widget type of your field to "Select list" at Manage Form Display of your content type
Check "apply chosen on this field" at Manage Fields > Edit of your field