Setting Field Visibility Based on a Dropdown Selection not showing

Hello Community, 

I am trying to make a Text field "Details" appear after click the option "Yes, conditionally" only. If this option is not selected then it would the Text field  "Details" will be hidden.

This is the code I am using and it is not working but I cant figure out why:

var nHide = (event.value=="Yes")?display.visible:display.hidden;
this.getField("Details").display = nHide;