form > .panel > .panel-body > form-group content is not inside the .panel-body

Here is my use case: I want to create panels inside a form in order to distinguish different criteria.

My problem is that when I create a <div class="form-group"> inside my <div class="panel-body">, the content of the form-group goes out of its boundaries: in the screenshot, my checkboxes are out of the panel-body boundaries.

Here is my DOM tree:

<form class="form-horizontal">
    <div class="col-md-6">
        <div class="panel panel-default">
            <div class="panel-body">
                <div class="form-group">
                    <div class="checkbox">
                        <label>
                            <input type="checkbox"

Can somebody tells me how to do that in the good way?