We walk in the dark places no others will enter
We stand on the bridge and no one may pass

SELECT  RIDER_NO
FROM    TRIP_DETAILS
WHERE   RIDER_NO NOT IN (SELECT RIDER_NO
                         FROM   RIDERS);

-- 1. add constraint but do not check existing rows
ALTER TABLE TRIP_DETAILS WITH NOCHECK
ADD CONSTRAINT fk_TRIP_DETAILS_RIDER_NO
FOREIGN KEY (RIDER_NO)
REFERENCES RIDERS(RIDER_NO);

-- 2. cleanup orphans as time allows...

-- 3. check the constraint to let the database engine record that it can rely on the existing data conforming to the constraint
ALTER TABLE TRIP_DETAILS CHECK CONSTRAINT fk_TRIP_DETAILS_RIDER_NO;