According to the U.S. Census Bureau, the county has a total area of 1,783 km² (689 mi²). 1,678 km² (648 mi²) of it is land and 106 km² (41 mi²) of it is water. The total area is 5.92% water.
As of the census2 of 2000, there are 18,643 people, 7,900 households, and 5,466 families residing in the county. The population density is 11/km² (29/mi²). There are 14,123 housing units at an average density of 8/km² (22/mi²). The racial makeup of the county is 97.63% White, 0.27% Black or African American, 0.59% Native American, 0.33% Asian, 0.02% Pacific Islander, 0.33% from other races, and 0.83% from two or more races. 1.44% of the population are Hispanic or Latino of any race.
There are 7,900 households out of which 23.60% have children under the age of 18 living with them, 58.50% are married couples living together, 6.70% have a female householder with no husband present, and 30.80% are non-families. 25.50% of all households are made up of individuals and 11.50% have someone living alone who is 65 years of age or older. The average household size is 2.33 and the average family size is 2.76.
In the county, the population is spread out with 20.80% under the age of 18, 5.60% from 18 to 24, 24.30% from 25 to 44, 28.40% from 45 to 64, and 20.90% who are 65 years of age or older. The median age is 44 years. For every 100 females there are 102.90 males. For every 100 females age 18 and over, there are 101.30 males.