PBGC paid $5.7 billion to more than 800,000 people in failed pension plans in 2015, comparable to what was paid out in 2014.

Single-employer plan program

In the single-employer plan, the deficit increase is largely explained by a decrease in the interest rate used to assess the cost of future liabilities.

The lower that interest rate, which is derived from a set of annuities offered in the private de-risking market, the greater the cost of future liabilities.

The so-called select interest rate factor decreased 55 basis points to 2.8 percent at the time of the report's accounting, down from 3.35 percent in 2014.

That translated to an increase of $4.7 billion in the cost of future liabilities. Another $3.3 billion increase resulted in a charge on the cost of accrued liabilities.

Those losses were offset by $4.1 billion in net premium income, also a new all-time high. Sponsors paid $3.8 billion in premiums in 2014, up from $2.9 billion in 2013.

Fiscal year 2015's numbers also suffered from slack returns in PBGC's investment portfolio, which returned $324 million, or 0.1 percent, well below the 1 percent return benchmark established in the Pension Protection Act of 2006.

The single-employer plan assumed responsibility for an additional 25,000 workers and retirees in 69 plans for the year. The terminated plans had an average funded ratio of 59 percent, and resulted in a total loss of $780 million to PBGC.

Five single-employer plans were newly classified as "probable" terminations, meaning PBGC expects the plans, which in total are underfunded by $422 million, to be insolvent in the next 10 years.

Multiemployer plan program

The nearly $10 billion increase in the multiemployer insurance program in fiscal year 2015 means the program, which protects the collectively bargained pensions of about 10 million union members, now has a more-than 50 percent chance of being insolvent in 2025.

The risk of insolvency then rises rapidly after the next 10 years, reaching over 90 percent by the end of the first 20 years.

When the program becomes insolvent, PBGC will be unable to provide financial assistance to pay guaranteed benefits in insolvent plans, the report said.

The decrease in interest rates also took their toll on the multiemployer program, to the tune of a $4.3 billion increase in the cost of projected liabilities.

But the identification of 17 new plans that recently terminated or are expected to go insolvent in the next year also resulted in $4.6 billion in increased liabilities, the largest portion of the nearly $10 billion growth in the multiemployer deficit.

The multiemployer program generated $212 million in premium income, and its investment portfolio returned $68 million.

The budget bill recently signed into law authorized three more years of premium increases in the single-employer plan, resulting in a per-participant flat rate of $78 by 2019. The fiscal year 2015 rate was $57.

Those increases are not accounted for in the new annual report and won't be accounted for until PBGC releases its next projection report.

Nor were the new provisions passed in the Multiemployer Pension Reform Act of 2014 accounted for in the annual report.

Those give the most critically underfunded plans the ability to reduce pension benefits to existing retirees as a last-ditch effort to stave off insolvency.

The report does, however, account for premium increases for multiemployer plans, which were doubled in the Multiemployer Pension Reform Act.

In a press conference, Thomas Reeder, PBGC's newly appointed director, noted the vital role the agency plays in securing the nation's private sector pension benefits. He also referenced the agency's high satisfaction numbers with beneficiaries.