.megasubmenu{ padding: 20px; }

@media (min-width: 992px){
    .dropdown-menu .dropdown-toggle:after{
        border-top: .3em solid transparent;
        border-right: 0;
        border-bottom: .3em solid transparent;
        border-left: .3em solid;
    }
    .dropdown-menu{
        margin:0;
    }
    .megasubmenu{
        left:100%; top:0; min-height: 100%; min-width:500px;
    }
    .dropdown-menu > li:hover .megasubmenu{
        display: block;
    }
}