document.onclick=closeDropDown;

var openDropDown="";

function dropDown(id){
    if(document.getElementById(id+'_dropdown').style.visibility=='visible'){
        document.getElementById(id+'_dropdown').style.visibility='hidden';
        openDropDown="";
    }else{
        document.getElementById(id+'_dropdown').style.visibility='visible';
        openDropDown=id+'_dropdown';
    }        
}

function closeDropDown(e){
    if(openDropDown!=""){
        document.getElementById(openDropDown).style.visibility='hidden';    
        openDropDown="";
    }
}