You need to add more to the second set of digits for the space. Imagine 1st set of digits as the total digits and second set is the part after the fraction there. So in your case you defined 15,15. As Lars said, you have not left any for the numbers before the fraction. Try 20,15, it works fine.
SELECT TO_DECIMAL(42242.640972,20,15) "to decimal" FROM DUMMY;
Even 19 works but not sure why? 