The Chiefs went back in front during the first half of the second quarter when Davis completed a 108-yard kickoff return, before Anthony Fasano caught Smith's 12-yarder to make it 21-7.
However, the Broncos pulled seven points back nearing the end of the second as Knowshon Moreno grabbed Manning's three-yard pass to leave the score at 21-14 going into half time.
Decker then levelled the score, before putting the Broncos ahead with another touchdown.
The Broncos man continued his impressive performance in the fourth, stretching the lead to 21-35.
Jamaal Charles pushed the Chiefs onto 28 points, but they were unable to avoid defeat.