This depends on many factors you haven't included. The exact BBs from you are important, but so are table dynamics, game format/structure, villain tendencies (player type), your position at the table, villain position at the table and so on.
Generally, open-shoving AT with 20 BBs seems a bit too loose. You are going to only get called by stronger hands most of the time. Similarly, If your opponent does have AK, AQ, or AJ, then you are dominated when you could have otherwise cut your losses.
If you are down to 10 BBs or less, then I'd say AT (especially if suited) becomes more reasonable to open-shove; 15 BBs or less if there is an ante in play.
However, I like your ambitious open-shove with 20 BBs and AT if you know your opponent is going to start folding out some of those hands like AJ or AQ. The problem here though is that most people aren't this tight and will probably call it off with AQ (and especially AK) a lot of the time. Again though, it depends on the tendencies of the villain (or your perception of their tendencies).
