That is a fairly large card in a relatively small chassis, but it should have decent airflow with that case.
Is the card overclocked? You could just set a lower power limit to reduce temperatures. You won't lose all that much performance either.
Idle average will be determined by the zero fan mode threshold, you should expect it to sit high when idle.
Make sure all your fans are correctly configured. Typical would be front fans as intake, rear and top as exhaust.
I think pictures are needed here. Just a description and general parts list isn't going to tell us all that much. Also going to need a complete parts list, including CPU and CPU cooler especially.