Get your vin# and go to a GM parts website - such as GMPARTSGIANT.COM) that does parts lookup by vin#, that will assure you have the correct part numbers, whether or not you purchase the parts from that website. My 2008 model has 2 idlers, smooth not ribbed, but yours may be different. Many websites will have an illustration of the various parts for the serpentine group. My favorite belt is a Gates Green Stripe fleet belt. Runs quiet, very dependable, does not produce static.
2008 Silverado LT, CCLB, Edge Insight CTS
2009 Arctic Fox 29-5T, Silver Fox Edition, Onan 3600 LPG generator