Hello,
You can display the IDOC data in t-code WE02. This only displays the list of field that are populated with some data.
For IDOC file, it depends on how your system is set up. If you are using any middleware like PI / XI, then IDOC would first go to PI and then to bank. So, you can ask your PI folks for the file. If the IDOC is directly sent to bank, then you can check with bank. If you are sending fiel as XML, then you can find the file in your system itself.
If you want to know the field mapping logic, you have to check the requirement documents. Most of the fields are populated by standard SAP by default unless there are some customer specific requirements.
Thanks,
V V