The IDMAP database can be dumped to a text file as shown here:

net idmap dump <full_path_and_tdb_filename> > dumpfile.txt

Where a particular build of Samba the run-time tdb files are stored in the /var/lib/samba directory the following commands to create the dump file will suffice:

net idmap dump /var/lib/samba/winbindd_idmap.tdb > idmap_dump.txt