Overflow: hidden nefunguje

#modalContainer{
  overflow:hidden;
}
#modalContainer {
  background-color:rgba(0, 0, 0, 0.3);
  position:absolute;
  width:100%;
  height:100%;
  top:0px;
  left:0px;
  z-index:100;
}
#alertBox {
  position:relative;
  width:300px;
  min-height:100px;
  margin-top:50px;
  background-color:#fff;
  position:relative;
  border-radius:10px;
  background-repeat:no-repeat;
  background-position:20px 30px;
  color:black;
  box-shadow: black 2px 5px 10px; 
  -webkit-box-shadow: black 2px 5px 10px;
  -moz-box-shadow: black 2px 5px 10px;
  filter: progid: DXImageTransform.Microsoft.Blur(PixelRadius='15', MakeShadow='true', ShadowOpacity='0.40');
}
#modalContainer > #alertBox {
    position:fixed;
}
#alertBox h1 {
    margin:0;
    font:bold 0.9em verdana, arial;
    background-color:#c8dd65;
    color:#FFF;
    border-top-left-radius:10px;
    border-top-right-radius:10px;
    padding:2px 0 2px 5px;
}
#alertBox p {
    font:0.7em verdana, arial;
    padding-left:5px;
    padding-right:5px;
    padding-bottom:0px;
}
#alertBox #closeBtn {
  background-color:#c8dd65;
    color:white;
    font-weight:bold;
    text-decoration: none;
    text-transform: uppercase;
    font-size: 14px;
  border-radius: 5px;
  border: 1px solid white;
  padding:5px;
  padding-right:20px;
  padding-left:20px;
  -webkit-transition: all 0.3s ease-in-out;
  -moz-transition: all 0.3s ease-in-out;
  -ms-transition: all 0.3s ease-in-out;
  -o-transition: all 0.3s ease-in-out;
  transition: all 0.3s ease-in-out;
}
#alertBox #closeBtn:hover {
  background-color:#9EB23F;
    text-decoration: none;
    color: black;
  border: 1px solid black;
  cursor: pointer;
}
#alertBox > #closeBtn {
  float: left;
  margin-left:117px;
  margin-bottom:5px;
}