Coolant spill?

You waited three whole minutes for an answer? It often takes many hours before someone qualified to answer reads what you posted, which isn't much. With no details or related history to go on, the logical answer is the reservoir is never supposed to be full. There's a "Min" line and a "Max" line on most models. If it's overfull, it is going to overflow shortly after a hot engine is stopped. The engine is still hot, but that heat is no longer being transported to the radiator to be cooled. As it heats up in the engine, that coolant expands and goes into the reservoir. There needs to be room in there for it. Once the engine cools down, the coolant contracts and the level will drop in the reservoir. As long as it remains near the "minimum" line, you do not add any.

One of two things. When a hot engine is stopped, circulation to the radiator stops, the coolant in the engine absorbs heat, then expands into the reservoir where it overflows if that reservoir was recently filled to the top. That might happen a few more times, but if there's no other defect, eventually enough liquid will be burped out. You'll find the level has gone down closer to the "minimum" line by as little as an hour or two later.

The next most common cause is coolant leaking from the hose going to the reservoir. They can develop rusted hose clamps, but more commonly they become dry-rotted. The rubber can also deteriorate if transmission fluid gets into the coolant. That's the result of the transmission cooler developing a leak. Those coolers are often inside one of the radiator's tanks. The hose will become slimy and mushy and you'll find the coolant in the reservoir is brown.

There's a third possible cause, but it involves the engine itself. It's not uncommon to develop a leaking cylinder head gasket in such a way that combustion gases get pushed into the cooling system. The pressurized gas pushes coolant into the reservoir rather forcefully. You may see bubbling in the reservoir while the engine is running.

When the head gasket leaks in this way, coolant can also leak into the affected cylinder. In this case you lose coolant instead of it overflowing. A cloud of white smoke from the tail pipe is the biggest clue. There's a chemical test for this. Rather than retype the explanation, start by looking at this article:

https://www.2carpros.com/articles/head-gasket-blown-test

Often you can borrow this tester from an auto parts store that rents or borrows tools, but they will make you buy your own bottle of special fluid. That's because it is rendered ineffective if it freezes or gets antifreeze in it. They don't want to risk borrowing it to you after the previous person contaminated the fluid. The liquid starts out dark blue. You draw air from the radiator through that liquid while the engine is running. If combustion gases are present, the liquid will turn a bright yellow. You can make it do that by breathing into it too. Draw fresh air through it again to make the fluid go back to dark blue and ready for the next test.

When you say coolant goes onto the floor, do you mean the garage floor or the vehicle's floor in the passenger compartment? Given the age of the car, a leaking heater core is a good suspect. Coolant typically drips on the passenger's feet, or in that area, but if the car came with air conditioning, there will be a condensate drain pan under the heater core. The common symptom then is coolant drips from a small rubber hose hanging down on the passenger side of the firewall, under the hood. You'll slowly lose coolant. It won't cause coolant to overflow the reservoir.

Here's a link to a related article:

https://www.2carpros.com/articles/head-gasket-blown-test

but you might want to look at the other dandy articles on this list:

https://www.2carpros.com/articles

Cooling system articles are quite a way down under "Engine Cooling".