Design Doc: Designing The World

Disclaimer: This is NOT my first design nor am I new to game development or project management, although this is my first open-world design.

So I am currently designing an Open-world RPG and I have finished an overview of the player story and world story. With this I have a broad map that the world will take place in that has a small grid on top of it to so the ingame sections and scale.

At some point during the design phase I need to actually layout the contents for the world withing each of these "zoomed in" grid cells (For reference on what I mean by the grid, refer to the Fallout maps where you can see the grid of the world). A building goes here, trees go here, zombies spawn here, Merchant here, etc. The question really becomes, how detailed shoukd the design be? Should it only be the buildings with basic inhabintants? Or should I try to design every quest before the actual world scenes are made in the engine?

Also, when you're designing a world, what is your typical workflow? How detailed do you make it, where do you start and what information do you typically include? (This is a personal question NOT a "give me a tutorial" question)

TL;DR: How do you personally design the world for an open-world RPG and what information do you include? Also, how detailed should it be, and how much should be added after the physical world is made?