As of the census of 2000, there were 19,830 people, 7,498 households, and 5,443 families residing in the county. The population density was 26 people per square mile (10/km²). There were 8,266 housing units at an average density of 11 per square mile (4/km²). The racial makeup of the county was 98.49% White, 0.11% Black or African American, 0.29% Native American, 0.22% Asian, 0.01% Pacific Islander, 0.35% from other races, and 0.55% from two or more races. 1.03% of the population were Hispanic or Latino of any race.
There were 7,498 households out of which 34.20% had children under the age of 18 living with them, 62.60% were married couples living together, 6.70% had a female householder with no husband present, and 27.40% were non-families. 23.60% of all households were made up of individuals and 11.90% had someone living alone who was 65 years of age or older. The average household size was 2.61 and the average family size was 3.11.
In the county the population was spread out with 27.90% under the age of 18, 6.30% from 18 to 24, 27.60% from 25 to 44, 22.90% from 45 to 64, and 15.30% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 99.10 males. For every 100 females age 18 and over, there were 98.10 males.
The median income for a household in the county was $42,173, and the median income for a family was $49,443. Males had a median income of $33,309 versus $22,922 for females. The per capita income for the county was $18,392. About 5.30% of families and 6.60% of the population were below the poverty line, including 7.30% of those under age 18 and 7.00% of those age 65 or over.