Are you going to start driving the car now? It'd probably be fine to run it (and way better that it was full of ethanol-free gas), but I guess I'd be inclined to siphon out what I could and top off with fresh fuel, just to avoid any trouble. I'd be less worried about water contamination than I would be about the fuel breaking down. You could also siphon out, say, two five-gallon jugs, top off with fresh fuel, and run one jug of old fuel with the next couple tanks of gas. That might be a compromise avoiding your having to get rid of fifteen gallons of questionable fuel - that's probably what I'd do. But you're more than likely fine to run it as is (it's unlikely to cause any problems whatsoever).
By the way, I had a 3800-powered Lacrosse for a few years; nothing like an old-school Buick for highway trips!