According to the U.S. Census Bureau, the county has a total area of 1,503 km² (580 mi²). 1,439 km² (556 mi²) of it is land and 65 km² (25 mi²) of it is water. The total area is 4.29% water.
As of the census2 of 2000, there are 360,767 people, 135,229 households, and 100,475 families residing in the county. The population density is 251/km² (649/mi²). There are 140,309 housing units at an average density of 98/km² (252/mi²). The racial makeup of the county is 95.77% White, 0.73% Black or African American, 0.22% Native American, 1.49% Asian, 0.02% Pacific Islander, 0.87% from other races, and 0.90% from two or more races. 2.63% of the population are Hispanic or Latino of any race.
There are 135,229 households out of which 35.40% have children under the age of 18 living with them, 64.80% are married couples living together, 6.80% have a female householder with no husband present, and 25.70% are non-families. 20.90% of all households are made up of individuals and 8.10% have someone living alone who is 65 years of age or older. The average household size is 2.63 and the average family size is 3.08.
In the county, the population is spread out with 26.30% under the age of 18, 6.80% from 18 to 24, 29.80% from 25 to 44, 25.10% from 45 to 64, and 12.00% who are 65 years of age or older. The median age is 38 years. For every 100 females there are 96.80 males. For every 100 females age 18 and over, there are 94.40 males.