I think it depends on the size of your team. If you are working more or less by yourself (like I do) I don't see a lot of value is going into a lot of detail in the design documents if the concept doesn't really need to be put across to other people to implement. It's more of a flexible guide for you to get your ideas out of your head and to work with over time until you have a fun prototype working that you can show to others. Then (this is the hard part, as you seem to already know) you polish it until it is good enough to sell.
Also, don't make a lot of levels until you are very happy with your prototype, as chances are your level file format will change at some point and you'll have to throw them all away.
Once you have a fun prototype, making the levels should be a lot more fun too.
Good luck, and stick with it!