function window_height() {
		var height = -1;
		if (window.innerHeight)
		{
			height = window.innerHeight
		}
		else if (document.getElementsByTagName)
		{
			var html = document.getElementsByTagName('html');
			if (html[0].offsetHeight)
			{
				height = html[0].offsetHeight;
			}
		}
		return height;
}

function doc_height() {
		var height = -1;
		if (document.getElementById('doc').innerHeight)
		{
			height = document.getElementById('doc').innerHeight
		}
		else if (document.getElementsByTagName)
		{
			if (document.getElementById('doc').offsetHeight)
			{
				height = document.getElementById('doc').offsetHeight;
			}
		}
		return height;
}

function position_at_bottom (el) {
	el_height = document.getElementById(el).style.height;
	if(window_height() > doc_height()){
		document.getElementById(el).style.top = (window_height() - el_height) + 'px';
	}
}