You could do this a few ways. You can always draw line in the Print event using the Line method. Since you already have vertical lines that you want to extend, it's easier to just make them longer to match the section height.
In the Format event for the Detail, at the very end of the code, just before the End Sub, add:
Me.LineLeft.Height = Me.Section(acDetail).Height
Me.LineRight = Me.LineLeft.Height
LineLeft and LineRight are whatever your names are for the two lines.