Problem:Insert_ display big numeric # db

Hi,
Please help. How do I insert and display a large numeric(24,6) – 18 digits to the left of decimal and six digits to the right of decimal? I used double data type to insert and display the value to and from the db2/400. Only 16 digits from the left of decimal is accurate and the rest is incorrect. For example:
the inserting value-123456789012345678.123456;
the value in db is-123,456,789,012,345,680.000000;
displaying the value is-1.2345678901234568E17.

Thank you,
Nhan Nguyen