WordPress Customizer - How to save list of checkbox of pages?

Question

I'm trying to create an option within the customizer that is basically a list of pages with checkboxes. I have that part working but I don't understand how to save it AND how to retrieve the saved data.

This is what I have so far...

class Customize_Pages_Control extends WP_Customize_Control {
    public $type = 'pages';
    public function render_content() {
        $pages = get_pages(); ?>
        <span class="customize-control-title"><?php echo esc_html( $this->label ); ?></span> 
        <?php
            foreach ($pages as $page) { // Loop through pages and add checkboxes 
        ?>
        <label>
            <input type="checkbox" value="<?php echo $page->ID ?>">
            <?php echo $page->post_title; ?>
        </label> <?php
        }
    }
}

$wp_customize->add_setting('list_pages' , array(
    'capability'    => 'edit_theme_options',
));

$wp_customize->add_control(new Customize_Pages_Control($wp_customize, 'list_pages', array(
    'label' => __( 'Add pages to slideshow' ),
    'section' => 'options_ga_code',
    'settings' => 'list_pages',
    'type' => 'pages',
)));