As of the census of 2000, there were 70,205 people, 24,356 households, and 18,778 families residing in the county. The population density was 197 people per square mile (76/km²). There were 24,883 housing units at an average density of 70 per square mile (27/km²). The racial makeup of the county was 95.95% White, 0.59% Black or African American, 0.18% Native American, 1.56% Asian, 0.01% Pacific Islander, 0.87% from other races, and 0.82% from two or more races. 2.55% of the population were Hispanic or Latino of any race. 44.3% were of German, 12.1% Norwegian, 7.1% Irish and 6.2% Swedish ancestry according to Census 2000.
There were 24,356 households out of which 45.20% had children under the age of 18 living with them, 66.40% were married couples living together, 7.30% had a female householder with no husband present, and 22.90% were non-families. 18.10% of all households were made up of individuals and 6.10% had someone living alone who was 65 years of age or older. The average household size was 2.84 and the average family size was 3.26.
In the county the population was spread out with 31.50% under the age of 18, 6.90% from 18 to 24, 34.70% from 25 to 44, 19.50% from 45 to 64, and 7.50% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 100.00 males. For every 100 females age 18 and over, there were 98.00 males.
The median income for a household in the county was $65,540, and the median income for a family was $73,577 (these figures had risen to $78,035 and $89,100 respectively as of a 2007 estimate). Males had a median income of $47,271 versus $32,107 for females. The per capita income for the county was $28,486. About 2.30% of families and 3.50% of the population were below the poverty line, including 3.60% of those under age 18 and 6.90% of those age 65 or over.