how can i retrieve data from FCKeditor

i tried to use:

function onSubmit()
{
var content_detail = document.getElementById("content_detail").value;
alert(content_detail);
}

to alert FCKeditor data
<script type="text/javascript">
var oFCKeditor = new FCKeditor( 'content_detail' ) ;
oFCKeditor.BasePath = "{$PATH_BASE}/admin/wysiwyg/";
oFCKeditor.Height = 500 ;
oFCKeditor.Create() ;
</script>

but i can't retrieve any data from it???!
beside post Data, how can i retrieve data in FCKeditor form?