Chalkey is correct, you need to use the src attribute to specify the page to be contained in the iframe. Providing you do this, and the document in the iframe is in the same domain as the parent document, you can use this:
var e = document.getElementById("id_description_iframe");
if(e != null) {
alert(e.contentWindow.document.body.innerHTML);
}
Obviously you can then do something useful with the contents instead of just putting them in an alert.