In 2015, the U.S. federal government spent more on healthcare than on Social Security for the first time. The Affordable Care Act’s expansion of Medicaid and the growing availability of subsidies for exchange plans are driving much of the higher spending.

Enrollment in the ACA’s insurance exchanges will hover around 13 million in 2016, the Congressional Budget Office said in an expanded economic report Monday, down from its previous estimate of 21 million but still above HHS’ most optimistic projection.