In the latest version of MVC and Identity there is no true best practice out on how to get the user and their ID yet. Therefore I have personally found adding a reference at the top of my controller allows me to get the UserId via the User.Identity class.
To get the currently logged in user ID.
Reason for this being a string is because in Identity 2 the Primary key is a GUID in a string.
string id = User.Identity.GetUserId();
Hope this helps someone.