html:

<div class="detail">
    <div class="detail-wrap">
        <div class="detail-main">
            <p>123</p>
        </div>
    </div>
    <div class="detail-close">
        <i class="icon-close">X</i>
    </div>
</div>

less: 

.detail {
    position: fixed;
    top: 0;
    left: 0;
    z-index: 2;
    width: 100%;
    height: 100%;
    overflow: auto;
    .detail-wrap{
        box-sizing: border-box;
        min-height: 100%;
        padding-top: 64px;
        padding-bottom: 64px;
    }
    .detail-close{
        position: relative;
        width: 32px;
        height: 32px;
        margin: -64px auto 0;
        font-size: 32px;
    }
}

DONE!