I usually do this with classes, that seems to force the browsers to reassess all the styling.
.hiddendiv {display:none;}
.visiblediv {display:block;}
then use;
<script>
function show() {
document.getElementById('benefits').className='visiblediv';
}
function close() {
document.getElementById('benefits').className='hiddendiv';
}
</script>
Note the casing of "className" that trips me up a lot