Details always appear on top of model geometry. However, you can change the draw order of details, placing them behind or in front of other details.
Every detail component, detail line, and filled region has a graphic draw order within the overall detail. Think of it as a stack of papers on your desk that overlap each other. You can send the detail component to the back of the detail. This is like taking the top paper and placing it at the bottom of the pile. You can also send the detail component back one step at a time. Think of this as taking the top piece paper and placing it behind the next piece of paper in the stack. Of course, you can do the reverse as well. You can bring the detail to the top of the stack or bring it forward one step at a time.