Why is it that after 15 minutes of idling the coolant starts to boil up and spill and the car begins to overheat?
It could be a couple of possibilities. One is that the coolant is not circulating correctly, maybe due to a blockage, and not able to cool down in the radiator. The other possibility is that the head gasket is breached, allowing combustion heat and gasses to overwhelm the cooling system.
Thermostat, water pump, coolant fan, clogged radiator, just to name a few causes.