body, html{
    position: relative;
    width: 100%;
    height: 100%;
}

.login_form{
    top: 50%;
    left: 50%;
    position: absolute;
    margin-top: -150px;
    margin-left: -160px;
}

.login_form .well{
    margin-bottom: 0;
}

.login_form .span4{
    float: none;
    width: 315px;
    margin-left: 0;
}

.login_form form input{
    width: 256px;
    margin-bottom: 15px;
}