Those numbers put GM among the largest pension sponsors. But a recent paper from Prudential argues that a borrow-to-fund strategy could benefit a "wide range" of plan sponsors, regardless of the size of plans or liabilities, or whether the plans are already frozen.

"Once a pension plan becomes well-funded, the sponsor faces an asymmetric risk-reward tradeoff—greater downside risk, and limited upside potential," write leaders of Prudential's pension risk transfer business. At the end of 2015, Prudential oversaw $60 billion in U.S. pension liabilities, by far the largest of any insurance company.

"Complementing plan design and investment strategy enhancements aimed at pension risk reduction, borrowing to fund is an integral tool for plan sponsors to use in the context of an overall risk-reduction strategy," write Scott Kaplan, Rohit Mathur, and Peter Kahn.

In borrowing to fund a pension, sponsors replace their potentially volatile annual contribution costs with a known, certain fixed-funding cost, says Prudential's team.

Moreover, last year's announcement of increased premium rates to the Pension Benefit Guaranty Corp., which will raise variable premium rates to 4.1 percent by 2019, can be neutralized by borrowing to fund liabilities.

To make its point, Prudential analyzes a hypothetical company's options. The imagined BBB-rated company sponsors a $7 billion plan that is 85 percent funded. Half of participants are retired, half are active participants and the average age of plan participants is 65. The average annual pension payment is $8,200.

The plan has already implemented a low-risk, liability driven investment strategy that limits assets' exposure to equities.

In issuing $1 billion in new debt to bring the plan to fully funded status, the hypothetical sponsor would realize $150 million in savings over a 10-year period, according to Prudential.

Without the debt, the sponsor would need to contribute $120 million over ten years to get to fully funded status. Along the way, it would pay pay $223 million in variable PBGC premiums. Together, those payments add to more than $1.4 billion in pension costs.

In issuing $1 billion in new debt, the BBB-rated company would pay $269 million in interest rates over 10 years, along with the $1 billion in principal payments.

After accounting for tax deductions specific to each scenario, Prudential arrives at the $150 million in savings realized under the borrow-to-fund example.

A key component of the borrow-to-fund strategy is realized from avoiding PBGC premiums, says Prudential.

"Over the last four years, PBGC premium increases have been included in three separate acts passed by Congress," reminds Prudential's team.

"As a result, the variable premium has increased by over 450 percent, from 0.9 percent of unfunded liability in 2013, to 4.1 percent in 2019. These increases impose a significant burden on plan sponsors with underfunded pension plans," wrote Prudential's team.

Eight other sponsors have initiated a borrow-to-fund strategy, notes Prudential. International Paper Co. also issued $2 billion in funding debt in 2015. In 2011, Raytheon Company issued $1 billion. CSX Corp. issued $300 million in debt, the lowest amount among the companies to have implemented the strategy, said Prudential.