Combine first 6 ingredients in a medium saucepan; bring to a boil. Cover, reduce heat, and simmer 20 minutes. Add milk and margarine, stirring until margarine melts.
Transfer half of potato mixture to container of an electric blender or food processor; cover and process until smooth, stopping once to scrape down sides. Return pureed mixture to saucepan; add caraway seeds. Cook over medium heat until thoroughly heated.