Are you sure that it's not adding E+12 (like 1.23456E+12)?

If it is, then this is scientific notation.  It means that 1.23456 is being
multiplied by 10^12 (or 1000000000000 (twelve zeros if I counted correctly).

You can format the cell as number with as many decimal places as you want.  

You could format the cell (whole column???) as Number first and then forget
about it, too.

Just one warning.  Excel's limit for significant digits is 15.  If you need more
(like when you're entering credit card numbers, you can either format the cell
as Text first or prefix your entry with a single quote ('123456).

These entries won't be treated as numbers by excel.