Have a read of:
FCOM DSC 22_30-10
and DCS 22_30-70-50
THR DES is only available in NAV mode and takes account of flight plan, constraints, cabin altitude and selected FCU ALT, to plot a suitable descent profile. The engines may be above idle to achieve this, so EWD highlights to you when the engines are actually at idle.
THR IDLE is just that, minimum thrust. If you are not in NAV but in HDG, and pull for descent, you will only get THR IDLE and you have to fly the descent profile you want via V/S or speed etc.