As of the census of 2000, there were 15,356 people, 5,772 households, and 4,086 families residing in the county. The population density was 26 people per square mile (10/km²). There were 6,024 housing units at an average density of 10 per square mile (4/km²). The racial makeup of the county was 95.57% White, 0.12% Black or African American, 0.26% Native American, 0.33% Asian, 3.09% from other races, and 0.62% from two or more races. 5.43% of the population were Hispanic or Latino of any race. 65.7% were of German and 6.3% Norwegian ancestry according to Census 2000.
There were 5,772 households out of which 33.60% had children under the age of 18 living with them, 61.10% were married couples living together, 5.80% had a female householder with no husband present, and 29.20% were non-families. 25.40% of all households were made up of individuals and 13.00% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.14.
In the county the population was spread out with 27.70% under the age of 18, 7.50% from 18 to 24, 27.10% from 25 to 44, 21.30% from 45 to 64, and 16.40% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 102.90 males. For every 100 females age 18 and over, there were 99.90 males.
The median income for a household in the county was $41,458, and the median income for a family was $48,923. Males had a median income of $31,002 versus $22,527 for females. The per capita income for the county was $18,004. About 5.10% of families and 8.10% of the population were below the poverty line, including 9.80% of those under age 18 and 7.80% of those age 65 or over.