As of the census of 2000, there were 16,571 people, 6,428 households, and 4,634 families residing in the county. The population density was 30 people per square mile (11/km²). There were 10,822 housing units at an average density of 19 per square mile (7/km²). The racial makeup of the county was 97.54% White, 0.21% Black or African American, 0.78% Native American, 0.22% Asian, 0.05% Pacific Islander, 0.10% from other races, and 1.10% from two or more races. 0.86% of the population were Hispanic or Latino of any race. 24.6% were of German, 12.4% English, 10.4% Irish, 10.0% American, 6.3% Polish and 5.1% French ancestry according to Census 2000. 98.8% spoke English as their first language.
There were 6,428 households out of which 31.70% had children under the age of 18 living with them, 58.60% were married couples living together, 9.00% had a female householder with no husband present, and 27.90% were non-families. 22.30% of all households were made up of individuals and 8.20% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 2.95.
In the county the population was spread out with 25.60% under the age of 18, 7.60% from 18 to 24, 28.60% from 25 to 44, 24.50% from 45 to 64, and 13.70% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 101.30 males. For every 100 females age 18 and over, there were 98.90 males.
The median income for a household in the county was $36,072, and the median income for a family was $39,932. Males had a median income of $31,860 versus $20,455 for females. The per capita income for the county was $16,309. About 8.20% of families and 10.50% of the population were below the poverty line, including 14.20% of those under age 18 and 7.00% of those age 65 or over.