「accordion__content」というクラス名の中に隠す内容を入れる。

<div class="accordionBox--type05">
  <details class="accordion">
    <summary class="accordion__button">
      <span>Q</span><p>項目タイトル 01 質問</p>
    <!-- /.accordion__button --></summary>
    <div class="accordion__content">
      <span>A</span><p>ここに質問の答えがはいります</p>
    <!-- /.accordion__content --></div>
  <!-- /.accordion --></details>

  <details class="accordion">
    <summary class="accordion__button">
      <span>Q</span><p>項目タイトル 02 質問</p>
    <!-- /.accordion__button --></summary>
    <div class="accordion__content">
      <span>A</span><p>ここに質問02の答えがはいります</p>
    <!-- /.accordion__content --></div>
  <!-- /.accordion --></details>
<!--//.accordionBox--></div>
                

.accordionBox--type05 {
  display:block;
  border-top:1px solid #DDD;
  .accordion {
    border-bottom:1px solid #DDD;
  }
  .accordion[open] {
    .accordion__button::before{
      background-size: 40% 8%, 0 40%;
    }
  }
  .accordion__button{
    display:block;
    position:relative;
    padding:1em 2.5em 1em 2em;
    cursor:pointer;
    &:hover p{
      color:#DDD;
    }
    &::before{
      content: "";
      box-sizing: border-box;
      position: absolute;
      right: 0.5em;
      top: 50%;
      width: 2em;
      height: 2em;
      transform: translateY(-50%);
      border-radius: 50%;
      border: 1px solid #DDD;
      background-image:
        linear-gradient(#000 0 0),
        linear-gradient(#000 0 0);
      background-repeat: no-repeat;
      background-position: 50% 50%;
      background-size: 40% 6%, 6% 40%;
      transition: background-size .3s;
    }
    >span{
      display:inline-block;
      font-size:1.1em;
      font-weight:bold;
      color:#666;
      position:absolute;
      top:0.8em;
      left:0.5em;
    }
    p{
      margin-bottom:0;
      font-weight:bold;
      transition:color 0.3s;
    }
  }
  .accordion__content{
    display:block;
    position:relative;
    padding:0 1em 1em 2em;
    >span{
      display:inline-block;
      font-size:1.1em;
      font-weight:bold;
      color:#AAA;
      position:absolute;
      top:0.1em;
      left:0.5em;
    }
    p{
      margin-bottom:0;
    }
  }
}