diff --git a/core/includes/form.inc b/core/includes/form.inc
index 540966c..9b4eb4f 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -233,6 +233,8 @@ function template_preprocess_fieldset(&$variables) {
  */
 function template_preprocess_details(&$variables) {
   $element = $variables['element'];
+  Element::setAttributes($element, array('id'));
+  Element\RenderElement::setAttributes($element);
   $variables['attributes'] = $element['#attributes'];
   $variables['summary_attributes'] = new Attribute();
   if (!empty($element['#title'])) {
@@ -247,12 +249,13 @@ function template_preprocess_details(&$variables) {
 
   if (!empty($element['#description'])) {
     $variables['description_display'] = isset($element['#description_display']) ? $element['#description_display'] : 'before';
-    $description_attributes = [];
-    if (!empty($element['#id'])) {
-      $description_attributes['id'] = $element['#id'] . '--description';
-    }
+    $description_id = $element['#attributes']['id'] . '--description';
+    $description_attributes['id'] = $description_id;
     $variables['description']['attributes'] = new Attribute($description_attributes);
     $variables['description']['content'] = $element['#description'];
+
+    // Add the description's id to the fieldset aria attributes.
+    $variables['attributes']['aria-describedby'] = $description_id;
   }
 
   $variables['children'] = (isset($element['#children'])) ? $element['#children'] : '';
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index cb66445..c901c9c 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -908,7 +908,6 @@ public function doBuildForm($form_id, &$element, FormStateInterface &$form_state
       '#required' => FALSE,
       '#attributes' => array(),
       '#title_display' => 'before',
-      '#description_display' => 'after',
       '#description_display' => (isset($element['#type']) && $element['#type'] == 'details') ? 'before' : 'after',
       '#errors' => NULL,
     );
diff --git a/core/themes/stable/templates/form/details.html.twig b/core/themes/stable/templates/form/details.html.twig
index f4c99e1..76e8c0f 100644
--- a/core/themes/stable/templates/form/details.html.twig
+++ b/core/themes/stable/templates/form/details.html.twig
@@ -25,7 +25,7 @@
     </div>
   {% endif %}
 
-  {{ description }}
+  {{ description.content }}
   {{ children }}
   {{ value }}
 </details>