Laravel 5 extendable entities

So, let's say I have basic Document with title, author,link_to_file properties and hasParentDocument and hasChildDocument recursive relations.

Now I want to extend my Document into, let's say, 3 or MORE types: Letter,Report and Contract, which can have their own unique properties (e.g. Letter has sender and reciever, Contract can have a some fields like valid_until and such) in the addition to inherited ones.

And I want them all to still have title property (inherited from Document) and be able to have child-parent relations with eachother (Contract can be parent of Letter, which can be parent of another Contract, which can be parent of Report... etc.) without hardcoding it.

How do I implement this in laravel? I mean the extending thing. I'm aware of the fact that I can implement Letter,Report and Contract as separate entities without having an ancestor class/entity.

via Chebli Mohamed