Pension plan funding levels among large publicly held U.S. employers soared last year as rising interest rates reduced liabilities, while strong investment results fueled a rise in the value of plan assets, according to a Milliman Inc. survey released Wednesday.

Defined benefit plans offered by the 100 U.S. employers with the largest pension programs were, on average, 87.9% funded at year-end 2013, up from 77.3% in 2012 and 79.1% in 2011.

In all, the market value of pension plan assets, aided by average investment returns of 9.9%, increased by about $68.5 billion to around $1.399 trillion in 2013, according to the survey.

The value of plan liabilities, deflated by higher interest rates, plummeted by nearly $130 billion to $1.592 trillion.

Combined, the rise in plan assets and fall in plan liabilities produced the biggest one-year improvement — $198.3 billion — in plan funding in the 14 years Milliman has been conducting the analysis.

In all, the plans' aggregate funding deficit in 2013 was $193.2 billion, down from $391.5 billion in 2012.

“Last year was a great year for pension funded status and helped reduce much of the underfunding that has persisted since the global financial crisis,” said John Ehrhardt, a Milliman consulting actuary in New York and co-author of the pension study, in a statement.

In fact, 18 of the 100 employers had plans that were at least 100% funded in 2013, up from just six in 2012 and seven in 2011.

Among surveyed employers, Armonk, N.Y.-based IBM Corp. had the largest defined benefit program with $93.4 billion in assets. Its plans were 93.7% funded last year, up from 86.4% in 2012.

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General Motors Co. had the second-largest pension program with $79.2 billion in assets last year, down from $83.6 billion in 2012. Benefit obligations fell to $99.0 billion in 2013, down from $111.4 billion in 2012. GM's plans were 79.9% funded in 2013, up from 75.1% funded in 2012, according to the Milliman survey.

The size of GM's pension program has dropped significantly in recent years. For example, in 2011, GM's plans had $108.9 billion in assets and $134 billion in liabilities.

That big fall in plan size is the result of twin actions GM took in 2012 to reduce the size of its pension plans through a de-risking approach.

One of those steps was an offer GM made to 44,000 salaried employees who retired after Oct. 1, 1997, and before Dec. 1, 2011, to convert their monthly annuities to a cash lump-sum payment. In all, GM paid about $3.6 billion to the 13,000 plan participants who accepted the offer.

As the second part of its pension de-risking strategy, GM purchased a group annuity from Prudential Insurance Co. of America for $25.1 billion. The annuity replaced benefits that about 76,000 salaried employees who retired before Oct. 1, 1997, received from GM, as well as retirees who declined the lump-sum conversion offer.

Nextera Energy Inc., a Juno Beach, Fla.-based energy and utility company, had the highest funded ratio in 2013 at 163.8%, up from 142.7% in 2012.

Atlanta-based Delta Air Lines Inc. had the lowest funded ratio — 46.9% in 2013 — up from 38.1% in 2012.

In conducting the analysis, Seattle-based Milliman analyzed financial reports of publicly held companies sponsoring the 100 largest pension programs for which full-year data was available.