<div id="right">right</div>
<div id="left">left</div>

#right {
  width: 200px;
  min-height: 300px;
  float: right;
  background-color: #CCC;
  border-left: 3px solid #357894;
}

#left {
  width: calc(100% - 203px);
  width: -moz-calc(100% - 203px);
  min-height: 300px;
  float: left;
  background-color: #CCC;
}