$1.1 billion isn't surprising. DW has gotten huge and is turning into a mega-resort, SDC is pretty big, and WA isn't anything to sneeze at either, and that's just the theme parks. Ride-the-ducks, the globetrotters, Dixie Stampede, Showboat, Stone Mountain, two major aquariums, the SF cable cars... it all adds up quickly. Then you start thinking about all their profitable partnerships including those with Coke, Humana, Rudolph, etc. The occasional side ventures they take on such as Grand Village and CNL management contracts only add to the pile.
I'm not sure if some of the other side businesses within the family like the butterfly palace play into this or not, that might be controlled by a separate part of the family than what we're talking about here.