How much Documentation to do in Agile?

In the Agile world, there is always a doubt about the amount of Test Documentation required. I’ve seen in various projects that it totally depends upon the Agile team, their understanding of Agile methodologies and project timelines. Sometimes, teams are doing too much of documentation and sometimes not at all. The best solution is to find the mid-way where the timelines are followed and the documentation is kept to a minimum level for a better understanding. For example, if one doesn’t have a Test Plan and just working on the Test Cases then like waterfall nobody knows what is the testing approach. It may not be a heavy document as normally in the Waterfall project but enough to cover the important testing areas.