There are a few ways to approach this.
One is to use a global Singleton object that contains the "ShowDetails" value. Every itemRenderer can access that object and change it's settings. Cairngorm's ModelLocator is one example of a singleton that is commonly used in this way. I believe swiz has something similar. You could also roll your own if needed.
You could try to extend the List class The List keeps itemRenderers in an array, which I believe is protected. You'll have to extend the List, and make this protected array public. Then you'll be able to access the list of itemRenderers and modify properties on them directly.
However, I'm not sure I would recommend either approach. An itemRenderer really should choose what to display based on the data it is displaying; not some global variable. Can you change the objects in your dataProvider and have the itemRenderer update accordingly? It is a third option; although I'm not sure if it is any better, or worse, than the previous two approaches.