As of the census of 2000, there were 17,163 people, 6,974 households, and 4,791 families residing in the county. The population density was 18 people per square mile (7/km²). There were 7,605 housing units at an average density of 8 per square mile (3/km²). The racial makeup of the county was 98.76% White, 0.11% Black or African American, 0.15% Native American, 0.35% Asian, 0.01% Pacific Islander, 0.29% from other races, and 0.34% from two or more races. 0.81% of the population were Hispanic or Latino of any race.
There were 6,974 households out of which 30.90% had children under the age of 18 living with them, 60.40% were married couples living together, 5.80% had a female householder with no husband present, and 31.30% were non-families. 28.70% of all households were made up of individuals and 15.50% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 2.98.
In the county the population was spread out with 25.80% under the age of 18, 6.10% from 18 to 24, 24.30% from 25 to 44, 23.60% from 45 to 64, and 20.10% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 95.30 males. For every 100 females age 18 and over, there were 94.00 males.
The median income for a household in the county was $34,562, and the median income for a family was $41,159. Males had a median income of $30,191 versus $20,184 for females. The per capita income for the county was $16,598. About 7.50% of families and 10.20% of the population were below the poverty line, including 12.40% of those under age 18 and 8.60% of those age 65 or over.