When was the thermostat last replaced?

There is a relationship between coolant temp and idle speed. If the t-stat is not opening/closing properly, that can be a cause.

There is thermo wax that controls a valve in the throttle body (TB) depending on coolant temp. Another cause can be plugged lines to/from the TB. Sometimes, the thermo wax & valve can be bad; like if the valve stays open.

For a temporary fix, you can always adjust the idle speed down w/ the screw on the TB.