To use the log out idle functionality a job is required to be scheduled in batch, program name:
/BTI/MDE_LOGOUT_USERS. This job should be scheduled as frequently as the log out idle time to ensure the system can pick up the idle users and log them out effectively i.e. if the log out idle time is 20 minutes the batch job should be ran every 20 minutes. If you run the job less frequently then that will increase the idle time.