Alternatives to Detention Act of 2018 This bill requires the Department of Homeland Security (DHS) to establish programs for providing alternatives to detaining individuals under the immigration laws, including community-based supervision and community support programs. DHS may not detain and must place in an alternative detention program individuals deemed members of a vulnerable population (e.g., asylum seekers, pregnant or nursing women, or crime victims), a parent of a child under age 18, or a caregiver. DHS must establish the position of Coordinator of Alternatives to Detention and Family Reunification to coordinate the use of alternative detention programs and to reunify parents with children separated from them.