Is it formal to inform readers that a point will be discussed later in the chapter? I mentioned an important idea in a section without developing it.

Is it formal to inform readers that the point will be discussed later in the chapter?

For example: 
This idea will be discussed in detail in a subsequent section.

If yes, should it be presented just like any sentence, maybe next to the idea? Or Does it require special treatment?

Thank you
There is nothing wrong with doing so. if you would not, the reader might start wondering. In cases where no other alternatives apply then signalling to the reader that more details will follow allows the reader to continue concentrating on the details at hand. If you end up with a situation like this, you should of course make sure that following such a structure is the best way forward. If it is not then restructuring the text is necessary. After all the goal of writing is to provide the reader with the easiest way to understand your writing (the way you want them to). If you find yourself using this form of signalling often, you should probably take a critical look at your writing and ask yourself why this happens. It might be that you are not structuring the text in a good way.