ISP will detect it and typically they will send you a letter saying "we know you downloaded _____ illegally, if we catch you again, we will cancel your service".
It's actually the studios that own the content you are torrenting that will seed the torrent and then collect a list of all IP addresses that connect to them, then they ask your ISP to shut your internet off.
You ISP doesn't actually care if you torrent, because if they cancel your internet they lose out on money.