Many ways to get the job done, as there are many airlines. Looking at the COPA website, they fly from major ports in the US. They don't show anything out of ORD. Only New York, IAD and LAX with a stop in Panama City to get to GRU. Since the OP would have to make a hop to another airport to connect from Detroit, at least from ORD or IAD there is a non stop to GRU on UA. But depends on what the OP's preferences are. COPA does sometimes have the lowest fares shown on sites like Orbitz coming out of IAD, but for me, the stop in Panama (which may or may not require an additional visa) plus long overall travel time is not worth the difference.