Inside your function, verify your span (JS code):
function toggle(self) {
var n = self.parentNode.childNodes;
for(var i = 0; i < n.length; i++) {
if (n[i].className == 'department') {
n[i].style.display = n[i].style.display == '' ? 'none' : '';
}
}
var mySpan = self.childNodes[0];
mySpan.innerHTML = mySpan.innerHTML == '[+]' ? '[-]' : '[+]';
}