This is the expected behavior for most documents.
Most of my session log entries show CPU of 0, 0.001, or 0.002.
The CPU number in the session log is probably calculated over the duration of the session, hence the small number.
You probably have your 'Maximum inactive session time:' set to 1800, which is why the Exit Reason is 'Expired due to idle' after 30 minutes. The majority of session I see have an Exit Reason of 'Socket closed by client'.
Hope this helps,