To start with, the 3 words:
Idiopathic means that the cause is simply unknown. It occurs and we do not have any explanation.
Progressive means that it can be expected to get worse over time. It generally does not stay the same or get better, but completely on its own the disease tends to worsen. Some diseases "progress" quickly while others take years and years.
Polyneuropathy means a disease involving many nerves of the peripheral nervous system (the nerves that control our muscles, sense pain, temperature, touch, and pressure and are found throughout our body EXCEPT in the brain and spinal cord.)
IPP can be caused by a variety of things including infections, carbon monoxide poisoning, lead exposure, and certain medications (none of the ones you are taking). For it to be progressive, there is usually an underlying disease - most commonly diabetes but other diseases can account for it too.
In your case, it is idiopathic because you have no known disease that would cause it. While this may sound strange, it does happen. It may, for example, be hereditary.
Your symptoms of IPP will likely progress. Symptoms can include tingling, burning, or weakness in the arms/hands or legs/feet, pins-and-needle feelings, a lose of position sensation causing balance problems. Even constipation or heart problems can occur.
The bot***** *****ne is that whatever your current symptoms are will likely be the same symptoms you have in the future. Your doctor is telling you he does not expect them to go away. By the same token, if you have no problems with constipation or heart problems now it is unlikely these will develop later from IPP.
Cymbalta is a terrific medication that, in many people with IPP, actually keeps the disease from progressing or dramatically slows down the progression. So if it is working for you and you are tolerating it well then don't forget to take it every single day - especially if your symptoms get better (because that is likely due to the medication and not the IPP going away!)