As of the 2010 United States Census, there were 40,123 people, 15,455 households, and 10,360 families residing in the county. The population density was . There were 17,796 housing units at an average density of . The racial makeup of the county was 74.3% white, 21.2% American Indian, 0.4% Asian, 0.3% black or African American, 1.0% from other races, and 2.8% from two or more races. Those of Hispanic or Latino origin made up 5.6% of the population. In terms of ancestry, 21.7% were German, 13.5% were English, 12.2% were Irish, and 7.7% were American.