New GM idler pulley.  This is the front belt idler pulley that mounts on the alternator bracket, and is a smooth pulley.  This is not the alternator pulley.  Fits the following:

06-14 6.0L/6.2L Cadillac Escalade
03-13 5.3L Avalanche

02-13 4.8L/5.3L/6.0L/6.2L Silverado/Sierra

03-14 5.3L/6.0L Suburban