As of the census of 2000, there were 36,776 people, 15,333 households, and 10,487 families residing in the county. The population density was 33 people per square mile (13/km²). There were 26,627 housing units at an average density of 24 per square mile (9/km²). The racial makeup of the county was 97.71% White, 0.33% Black or African American, 0.66% Native American, 0.30% Asian, 0.05% Pacific Islander, 0.21% from other races, and 0.75% from two or more races. 0.66% of the population were Hispanic or Latino of any race. 44.4% were of German, 8.8% Polish, 7.9% Irish, 5.2% Norwegian and 5.2% English ancestry according to Census 2000.
There were 15,333 households out of which 27.00% had children under the age of 18 living with them, 57.80% were married couples living together, 7.10% had a female householder with no husband present, and 31.60% were non-families. 26.40% of all households were made up of individuals and 12.00% had someone living alone who was 65 years of age or older. The average household size was 2.34 and the average family size was 2.82.
In the county, the population was spread out with 22.30% under the age of 18, 5.70% from 18 to 24, 26.50% from 25 to 44, 26.80% from 45 to 64, and 18.70% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 99.20 males. For every 100 females age 18 and over, there were 97.30 males.