To add to Matthew's post, details cannot have the same ID (unlike a Section/Elevation, which I personally have mentioned to GS as somewhat of a downfall).
As shown in the attached image, a detail that is linked to a marker (whether the marker is in Plan or Section/Elevation) are easily identified in the Project Map, as the icons have a square border around them.
In the case of deleting a detail, you can simply right click on it (or if you're unfortunate enough to be living in a one-button-mouse world, then ctrl+click) and choose to 'Select Detail marker on the Source View and zoom to it'. Then all you need to do is hit the delete key. If you have more than one marker linked to the same detail, it will select the original. Once the original is deleted, the icon in the Navigator will appear with a circular border instead of square. [For some reason we are unable to right click again and select the non-original markers. Would be nice to select these too, preferably in order of creation IMHO].
Once all markers for a detail are deleted, the icon in the Navigator will appear with no border around it. These are invaluable for a comprehensive CAD Template!
Hope that demystifies detail manipulation for you!
Cheers,
Link.
