You sure thats 20k in profits? How would they even know the 20k you are cashing out is profits? Or you mean if you cash out 20k using coinbase? Someone that is cashing out 20k from coinbase could easily be down money on other exchanges etc. So how would they base the 20k on?
I am not sure if its profits, but I just logged into Coinbase and a popup announcement came up "don't forget to pay your taxes". Then when you click on it, it goes to a page with their tax policy, and it clearly states the $20,000 figure that it will be reported to the IRS. I can dig up the link for you later but take my word for it. I am in the 4 figures of trades/profits anyway. I made a lot more gains on altcoins, like way more than anything on Coinbase/GDAX.
Anyone else have any insight on this?