As of the census of 2000, there were 21,033 people, 9,066 households, and 6,300 families residing in the county. The population density was 24 people per square mile (9/km²). There were 22,397 housing units at an average density of 26 per square mile (10/km²). The racial makeup of the county was 89.69% White, 0.20% Black or African American, 9.08% Native American, 0.18% Asian, 0.01% Pacific Islander, 0.19% from other races, and 0.65% from two or more races. 0.86% of the population were Hispanic or Latino of any race. 37.8% were of German, 7.9% Polish, 6.6% Irish and 5.3% English ancestry according to Census 2000. 95.9% spoke English, 1.3% Spanish and 1.2% German as their first language.
There were 9,066 households out of which 23.40% had children under the age of 18 living with them, 58.40% were married couples living together, 7.50% had a female householder with no husband present, and 30.50% were non-families. 26.00% of all households were made up of individuals and 12.60% had someone living alone who was 65 years of age or older. The average household size was 2.29 and the average family size was 2.73.
In the county, the population was spread out with 20.70% under the age of 18, 5.00% from 18 to 24, 23.10% from 25 to 44, 28.50% from 45 to 64, and 22.80% who were 65 years of age or older. The median age was 46 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 95.80 males.