The main focus of CBO's long-term budget outlook is rightly on the unified budget numbers regarding spending, revenue, deficits, and debt. But it is also important to look at trust funds, both in what CBO estimates for their insolvency date and how CBO's assumptions about trust funds can affect debt.
CBO's ten-year projections also project insolvency dates for three trust funds: the Highway Trust Fund (later this summer), the Social Security Disability Insurance (SSDI) trust fund (FY 2017), and the Pension Benefit Guaranty Corporation's (PBGC) multiemployer pension fund (FY 2024). The PBGC's trust fund exhaustion is reflected in the budget numbers, meaning that spending is automatically limited to incoming revenue, but the other two much larger trust funds are assumed to continue spending at scheduled levels despite not having the resources to do so.
The same goes for the two trust funds whose exhaustion dates will come after ten years and thus are only discussed in the long-term outlook: the Medicare Hospital Insurance (HI) trust fund and the Social Security Old Age and Survivors' Insurance (OASI) trust fund. CBO does not specifically project an insolvency date for HI, which finances Part A of Medicare, because it doesn't do long-term projections for each part of Medicare. It does say that exhaustion would likely come shortly after ten years, and by the looks of the ten-year projections, that date would likely be around 2027.
As for OASI, CBO projects the trust fund to run out in 2031, the same as it projected last year. But on a combined basis, the Social Security trust funds would be depleted in 2029, one year earlier than CBO projected last year.