protected function filterAddInstitutionList($list)
    {
        foreach ($list as $entityId => $idp) {
            if (in_array($entityId, $this->whitelist)) {
                unset($list[$entityId]);
            }
        }

        if (empty($list)) {
            throw new Exception('All IdPs has been filtered out. And no one left.');
        }

        return $list;
    }