Has there ever been a non-trivial, stable, and functional anarchy?

Sure there are lots of small groups that operate on personal relationships and don't have what could be called a government. I don't dislike such things. I just worry that they are not candidate models for a country big enough so that everybody does not know everybody personally. And lots of anarchic conditions arise through some kind of war or disaster or some such, but don't last very long, or are very grotesquely bad places to consider living.

Let non-trivial be defined as roughly the size of or bigger than Iceland in 
population.  (About 350,000.) Or 100,000 for individual ancient cities if you have an example.

Let's be a little relaxed about the definition of functional. Maybe something like, if the country you currently live in was like that, you would not find it worth while to move to get away from it. This feeds into the stable thing. If a place is so horrible nearly everybody wants to leave then it can't be particularly stable.

Let stable be defined as at least 25 years with substantially the same arrangement. That is chosen to be about one generation.

So, no kings, no oligarchs, no junta, no elected law makers, no priestly leaders handing out punishment for transgressors. Only all voluntary, all the time. Anything like that?