Actually, you see the line item pertaining to vendor in FBL1N. when it comes Customer and GL, it is the same. So, if you want to see all the line items of a single document, then you have to create a SAP Query through SQVI by using tables of BKPF/BSEG or FAGLFLEXA or go for Z report.
If it is for tentative purpose, you can use the table BSEG.
Thanks