public function verifyIdToken($idToken = null)
  {
    $tokenVerifier = new Google_AccessToken_Verify(
        $this->getHttpClient(),
        $this->getCache(),
        $this->config['jwt']
    );

    if (null === $idToken) {
      $token = $this->getAccessToken();
      if (!isset($token['id_token'])) {
        throw new LogicException(
            'id_token must be passed in or set as part of setAccessToken'
        );
      }
      $idToken = $token['id_token'];
    }

    return $tokenVerifier->verifyIdToken(
        $idToken,
        $this->getClientId()
    );
  }