How to cite computer terminals like Bloomberg or Thomson Reuters? Consider that I looked up some data, more specifically credit ratings for a sovereign, in the Bloomberg and the Thomson Reuters terminal for my thesis:


  The sovereign is rated Aaa by Moody's, AAA by Standard and Poor's, and AAA by Fitch (Bloomberg).


What is the best practice to cite a computer terminal as the source after using data from that terminal? I mean there is no specific author or authoring date other than "Bloomberg" and the date that I've accessed it. And just stating "Bloomberg" as above looks odd.

This question is not directly related to any citation style, however I'm using BibTeX with the APA style for my thesis.
Unless there is a unique, persistent link to the statement from the rating agency - e.g. a link to their archive of ratings, etc, you cannot use that information as a citation. You should imagine the reader far in the future, reading and trying to verify your work - what would they consult, if anything in order to validate your statements, or reproduce your work. 

If the agencies do not maintain a public archive, you may be able to take a screenshot of the terminal, and publish it to a place which will keep a persistent public archive of it, e.g. figshare.