Thank you Bob B. for the ID of Tabanus atratus, male

This was about 1" long - that doesn't sound very large, but it sure was the biggest fly I've ever seen. I've since learned that female horseflies give very painful bites. I don't know how to tell the difference between the sexes, so I'm very wary with these seemingly peaceful slow moving creatures (at least when they're on my deck railing.)