The overarching outline is the broad strokes. It can be nothing more than a bulleted list. Each bullet should have only a couple of sentences. It should be primarily structural. In other words, you do not need to write a lot down in the overarching outline. Most of your ideas will change as you write, so do not be too specific in the overarching outline. You will wind up changing it anyways. What you do need to do in the overarching outline is set up the book’s structure. It is like drawing a map that leads your characters from the prologue to the epilogue. It should say where they go and what they do, who they meet, and what they learn along the way. What they learn is critical. I found myself facing situations where I realized, as I was writing, that my characters were supposed to know something that they did not know. Then, I had to go back five or six chapters and add a part where they learn this thing that they were supposed to know. If I had been more thorough in my overarching, structural outline, then I would not have forgotten when they needed to know what.
In the end, outlining actually saves you time. It makes you more efficient. More outlining is always better. It reduces writer’s block. It allows you to put down an idea for weeks or months and then pick it back up later. You cannot keep it all in your head! Write it down ahead of time! Going back and making changes to your book five or six chapters ago is hard. It makes you re-think everything about those chapters. You cannot always just throw in a critical scene. Sometimes, that critical scene ruins other parts of your plot. Then, you have to start reworking multiple parts of the story. In the end, things would have been simpler if you had spent more time on the front end, preparing your outline. Your outline will guide you through these pitfalls.