Ventas Inc. has reached an agreement to acquire for $3.1 billion “substantially all” of the senior housing assets of Atria Senior Living Group.

Chicago-based Ventas, a real estate investment trust, will dole out $1.5 billion in cash and stock and assume $1.6 billion in debt to acquire 118 projects in New York, New England and California. Atria’s management company will continue to oversee the properties.