According to a government report,

...based on estimated gains in health insurance coverage in 2014, the estimated unreimbursed cost of the care provided by hospitals to people who are uninsured or underinsured was $27.3 billion in 2014, $7.4 billion lower than it would have been if coverage had continued at its 2013 level, at $34.7 billion.

Source: "INSURANCE EXPANSION, HOSPITAL UNCOMPENSATED CARE, AND THE AFFORDABLE CARE ACT," U.S. Department of Health and Human Services, March 23, 2015, http://aspe.hhs.gov/health/reports/2015/MedicaidExpansion/ib_UncompensatedCare.pdf