BCP did not preserve relational data
You're lacking the -E flag for identity columns, which allows you to preserve the values instead of re-seeding them on the import.

bcp DocControl.dbo.Documents in Documents.dat -n -E -S SERVER\SQLEXPRESS -T


From MSDN,


  [-E] Specifies that identity value or values in the imported data file
  are to be used for the identity column. If -E is not given, the
  identity values for this column in the data file being imported are
  ignored, and SQL Server automatically assigns unique values based on
  the seed and increment values specified during table creation. 
  
  If the
  data file does not contain values for the identity column in the table
  or view, use a format file to specify that the identity column in the
  table or view should be skipped when importing data; SQL Server
  automatically assigns unique values for the column.