Even when at idle the amount of heat generated when an engine is higher in the RPMs and under load is a huge difference. When you get a chance after it has reached operating temperature, grab the upper radiator hose and see if it has pressure and is resistant to be pinched cloed with a slight grip. . When engine is cool check level. You might try getting a new radiator cap if you have a temperature gauge and it does not indicate an aoverheating situation.
I am giving you a walk through diagnostic that will cover all the bases and go into better detail and walk you through tests and symptoms.