Your function seems to be suffering from missing curly brackets and semicolons. Try this instead and it will work:
function hideshow(which)
{ if (!document.getElementById) { return; }
//
if (which.style.display=="block")
{ which.style.display="none"}
else { which.style.display="block"; }
}
//
You can simplify the if/else block by using a ternary operator instead:
var result=(which.style.display=="block")? "none" : "block";
which.style.display=result;