WITH GROUPS(CUST_ID, TOT_AMT) AS
  (SELECT CUST_ID, TOT_AMT
   FROM  ACTIVITY
   UNION ALL
   SELECT C.CUST_ID, A.TOT_AMT
   FROM   GROUPS A, 
          CUST_PARP_GRP_BIL B, 
          DFN_GRP_BIL C
    WHERE A.CUST_ID = B.CUST_ID
    AND   B.GRP_ID  = C.GRP_ID)
  SELECT CUST_ID, 
         SUM(TOT_AMT) AS TOT_AMT
  FROM    GROUPS
  GROUP  BY CUST_ID
  ORDER  BY CUST_ID;