If you need to reference a dynamically generated value you can also append query string paramters after the @URL.Action like so:
var id = $(this).attr('id');
var value = $(this).attr('value');
$('#user_content').load('@Url.Action("UserDetails","User")?Param1=' + id + "&Param2=" + value);
public ActionResult Details( int id, string value )
{
var model = GetFooModel();
if (Request.IsAjaxRequest())
{
return PartialView( "UserDetails", model );
}
return View(model);
}