What happens if you disconnect idle control stepper motor?

I don't know what sort of vehicle you have - most engines use conventional brush-type motors for idle air control. Those go to minimum air when disconnected because motor just works against a spring that tries to keep the valve closed. If your vehicle has a stepper motor it will just stay where it stopped. One of my sons once had an ancient Taurus that apparently had a weird arrangement where DC voltage opened the IAC and AC voltage from an inverter closed it, so if the inverter failed the idle would only increase.