Shelter Island, located on Flathead Lake in Montana, is owned by Donald G. Abbey, CEO of the Abbey Company. His newly-constructed mansion is on the southern tip.
Sold in 2016 to Howard Epstein, National Executive of Bank of America. He paid a reported $23.5M in April 2016, after originally being listed for sale for $78M.
http://www.hurwitzjamesco.com/property.php?lng=en&id=103