As per the KB 820729
Timer_ConnectionIdle : The connection expired and remains idle. The default ConnectionTimeout duration is two minutes.
Timer_ConnectionIdle, happens in normal HTTP protocol usage, where the client decides not to disconnect from the server because there is a good chance that it will have another request for the server either in the process of loading a page or because a client will probably click a link on a web page that will go back to the same server. By default the server will close the connection and reclaim those resources after 2 minutes of inactivity. This is nothing to worry about, it's just an informational type of entry.
I hope you do have 180 seconds of default timeout setting configured in IIS. You can use the article Configuring HTTP Server API Error Logging to completely disable HTTP.sys logging (not recommended) but there is no way to disable a particular event.