Permission wise you can look at an idea in the same way as a poll.
- View permission will only allow you to view an idea
- As of Contribute permission level you'll be able to vote on an idea
Haven't found any documentation on this either ...
Thank you for your helpful answer as always, Günter Van der Donckt! Luigi Mariani, Günter is correct in that the idea permission levels are similar to polls. In this documentation: Standard Space Permission Levels. The documentation points you to the admin console where you can see a better description of the permission levels, including ideas.
In the admin console, navigate to Permissions > Space Permission Levels and click on the Standard Levels tab. Click on whichever permission level you would like to look at and then you will see a comprehensive list: