help formatting a string to number

I have an input file with field the has a field description of 12v6 (12digits)implied decimal (6 digits) If I look at the data it looks like this 555555555555123456.
I am trying to load this into an oracle db. The field on the table is also 12v6. I am getting an oracle error because the the field is being loaded with all 18 digits to the left of the decimal.
How do I format it as 555555555555.123456 thanks Received on Sat Sep 12 1998 - 08:49:18 CDT