I just spent an hour on the phone with Customer Support trying to get a straight answer to this. I was told maybe there's a bug with my account, and that that feature is not offered. Ultimately I discovered that apparently the only way to do this is to go to Statements -> Custom and generate a CSV report. This is the only type of report that contains the balance. You can import a CSV file into Excel or whatever spreadsheet you have.
It is unconscionable that a supposed financial institution would make it so difficult to see this information. It should be part of every report.
