As of the census of 2000, there were 64,417 people, 21,581 households, and 16,746 families residing in the county. The population density was 148 people per square mile (57/km²). There were 22,827 housing units at an average density of 52 per square mile (20/km²). The racial makeup of the county was 96.73% White, 0.85% Black or African American, 0.45% Native American, 0.58% Asian, 0.02% Pacific Islander, 0.43% from other races, and 0.95% from two or more races. 1.10% of the population were Hispanic or Latino of any race. 40.1% were of German, 13.6% Norwegian, 7.5% Swedish and 6.2% Irish ancestry according to Census 2000.
There were 21,581 households out of which 44.90% had children under the age of 18 living with them, 66.20% were married couples living together, 7.50% had a female householder with no husband present, and 22.40% were non-families. 15.70% of all households were made up of individuals and 5.20% had someone living alone who was 65 years of age or older. The average household size was 2.91 and the average family size was 3.27.
In the county the population was spread out with 30.90% under the age of 18, 9.60% from 18 to 24, 33.90% from 25 to 44, 18.40% from 45 to 64, and 7.10% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 104.30 males. For every 100 females age 18 and over, there were 103.20 males.
The median income for a household in the county was $57,014, and the median income for a family was $61,790. Males had a median income of $41,601 versus $27,689 for females. The per capita income for the county was $21,322. About 2.30% of families and 4.40% of the population were below the poverty line, including 3.50% of those under age 18 and 10.10% of those age 65 or over.