You say "It has a lean condition and a rich condition codes"

If conflicting codes happen you want to observe the "Freeze Frame" data on your scanner to look at engine data from when each code was thrown.

Engine rpms, temp, load, etc are important clues.

Let's discuss what you've told us.

The computer is subtracting fuel at idle (-20% LTFT) but at higher rpms the LTFT returns to normal.

It thinks the engine is running rich at idle but not so at higher rpms.

Is it being lied to by a sensor, MAF, MAP, ECT, IAT, or O2 sensor or is it something else?

Since the condition is happening at idle that's easy enough to see with your scanner.

(Live Data) Look at those sensors. Check MAF, MAP, ECT, IAT with KEY ON first, without starting your Fiesta.

See if they look reasonable for the ambient temperature and static (engine off) condition.

Then start the Fiesta and observe them during warm up. See if they look reasonable again.

MAF g/s at idle. You didn't say which engine that Fiesta has but I think they had a tiny 1.3L engine. So the MAF sensor should be reporting around 1.3 g/s at idle.

If the MAF sensor is "overreporting" the amount of air entering the intake, the computer will add too much fuel and the upstream O2 sensor will see the rich condition and the result will be Negative fuel trims.

That's easy enough to see on your scanner.

Then, is it the O2 sensor lying at idle? Again, that's easy enough to see on your scanner.

Look at your fuel trims at idle. Then pull a vacuum line off of the intake manifold to force a lean condition and see if the fuel trim starts heading towards Positive (your -20 LTFT decreasing towards 0 as STFT goes Positive).

If it does you know your upstream O2 sensor is working.

Then maybe consider the EGR valve isn't closing all the way.

A "leaking" EGR valve is similar to a vacuum leak in as they both have more of an affect on an engine at lower rpms.

But a leaking EGR valve allows exhaust gases into the combustion chambers when the computer doesn't know about it and unlike a vacuum leak, exhaust gasses aren't combustible.

Without going into a long-winded explanation about why it happens, it will cause a rich condition. 

If your scanner can show you COMMAND EGR (EGR_PCT) and EGR ERROR (EGR_ERR) you can get a clue if that's happening.

 If everything checks out OK, then maybe you have a fuel injector that's leaking a little.

Hook up a fuel gauge. Turn the Key to ON to pressurize the fuel rail. Then turn the Key OFF and watch the fuel gauge to see if the pressure is dropping too quickly because an injector is leaking.

So here's the drill:

1) Observe the sensors, Key ON and then again during warm up.

2) Observe fuel trims as you force a lean condition at idle.

3) Observe EGR data.

4) Observe fuel pressure drop.

Partially clogged fuel filter (change it) or dirty injectors (clean or replace them.)

Clean your MAF sensor and throttle plate with the right cleaners. Run fuel injector cleaner as @Doc says, then recheck.