Trying to show or hide elements based on when a user selects a certain radio form input. What i have below doesn't seem to work any suggestions.

Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Untitled Document</title>
<script type = "text/javascript" language = "Javascript">
	function Proof(obj) {
		if (document.getElementById('proof')radios.checked.value == "no") {
			displayValue = "block";
		} else {
			displayValue = "none";
		}
		document.getElementById('proof_type').style.display = displayValue;
	}

</script>
</head>

<body>
<form action="" method="post" enctype="multipart/form-data">
<input name="proof" type="radio" id="proof" tabindex="1" value="yes" onchange="Proof(this);" /> Yes
<input name="proof" type="radio" id="proof" tabindex="2" value="no" onchange="Proof(this);" /> No
<div id="proof_type" style="display:none;">Proof Type</div>
</form>
</body>
</html>