For all the hoopla about the ACA exchanges, it appears that Medicaid accounts for the lion's share of coverage gains

See Be skeptical about coverage numbers in health-care debate by Doug Badger of the Galen Institute. Excerpts:

"The "20 million" figure appears to have originated in a March 2016 report by the U.S. Department of Health and Human Services (HHS). That report declared that "the provisions of the ACA have resulted in gains in health insurance coverage for 20.0 million adults through early 2016."
The precision of the figure - 20.0 million, not 20.1 or 19.9 - suggests a level of certitude that the report doesn't actually deliver. Its authors analyzed a blend of data from the government's National Health Interview Survey (NHIS), and the private Gallup Healthways survey. They concluded that 17.7 million nonelderly adults gained coverage between January 2014 and February 2016. The agency also estimated that 2.3 million young adults had gained private coverage between 2010 and 2013 because the ACA required employers to cover dependent "children" until their 26th birthdays.

Since the release of the HHS study, the government has published two additional surveys of health-insurance coverage - the Current Population Survey (CPS) and the American Community Survey (ACS). Both offer data through the end of 2015, allowing for comparison with the NHIS estimate.
The three surveys use different methodologies to produce estimates of the number of nonelderly adults who gained coverage. These estimates vary by 20 percent - ranging from the CPS estimate of 13.7 million to the NHIS estimate of 16.5 million.

Additional estimates by the Heritage Foundation's Edmund F. Haislmaier and Drew Gonshorowski fall near the lower end of that range. Their study found that 14 million people (including children) gained public or private coverage in 2014 and 2015. Unlike government surveys, Haislmaier and Gonshorowski examined data from insurance company regulatory filings and from the government's own headcount of Medicaid enrollment.

They found that 84 percent of the newly insured gained coverage through Medicaid and a related government program for low-income children. Another study attributed much of these gains to the ACA's expansion of Medicaid to nondisabled, non-pregnant adults. The authors also found that nearly half the new Medicaid enrollees met eligibility standards that were in place before the ACA.

For all the hoopla about the ACA exchanges, it appears that Medicaid accounts for the lion's share of coverage gains and that many new Medicaid enrollees would have been eligible for that program even if the ACA had never passed.

Then there's the HHS estimate that 2.3 million young adults gained coverage between 2010 and 2013 because the ACA requires employers to cover dependents age 19 to 25. HHS failed to mention that private coverage declined for every other age group over that period, falling to the lowest level in history. The biggest drop in private coverage occurred among children 18 and younger. By the end of 2013, 1.2 million fewer children were covered under their parents' policies than in 2010."