Ascension Health is the country's largest nonprofit hospital system, and says its mission is to serve all. In 2007, an Ascension subsidiary closed Riverview Hospital, the third hospital it has shut down in Detroit in the past 10 years. Meanwhile, 30 miles away, in a suburb of multimillion-dollar homes, Ascension is opening a new $224 million hospital. The approach is an increasingly common strategy among nonprofit hospital systems: Close money-losing facilities in poor areas where a large share of patients are uninsured, and build or refurbish hospitals in affluent places where people have private insurance coverage.