Comforter
A comforter is a soft flat bag filled with filled with polyester batting or other artificial/natural material and used on a bed as a type of blanket. A comforter usually doesn't cover the pillows or box spring of the bed and are mostly decorative, because they don't provide much warmth.
A similar type covering made from down and/or feathers is generally referred to as a duvet.
See also: Silk comforter