Fuel trims and wandering idle

[Radman]

I played with my ODBC dongle the other day and noticed my long term fuel trims are at approx -11% for both banks. I also noticed my idle bounces around. It will be at ~900rpm, drop down 600-700rpm and seem rough, then slowly rebound to around 900. Rinse and repeat.

If my understanding of the fuel trims are correct, my car thinks I'm getting too much fuel and is therefore running it lean at times. Would that explain the wandering idle?

If it does, then what should I be hunting for in regards for the reason of the lean fuel trim as my understanding is it should be much closer to 0.

Please be gentle, this is my first time with fuel trims lol!

TIA
Jason