The states with the steepest declines in uninsured rates last year tend to have one thing in common: participating in Obamacare's Medicaid expansion.

This chart from Gallup shows the places that experienced the biggest declines in numbers of uninsured residents. Nine of the top 10 states expanded Medicaid — and the decline in the uninsured rates in Arkansas and Kentucky are especially staggering.

Obamacare's expansion of private insurance gets way more attention than Medicaid, but the public program is arguably having a more significant impact on driving down the number of uninsured Americans. The Obama administration estimates that the program's enrollment has increased by 11 million people since January 2014. Medicaid now covers 69 million people — more than one in five Americans.

The one outlier on this list, its worth noting, is Montana: it's the only one in the top 10 that did not participate the Medicaid expansion. But its still had a 4.9 percentage point decrease in the state uninsured rate.

I can't find any especially good explanation of why Montana's uninsured rate declined so quickly. The state hasn't done an especially good job on outreach for its health insurance exchange; only about 29 percent of those eligible to use the marketplace actually have, which is pretty much average for the country. For now, it's just the Montana mystery.