When General Motors Co. announced that it was purchasing a group annuity from Prudential Insurance Co. of America to replace pension benefits that tens of thousands of former long-time salaried workers received from the automaker, it was not the first employer to take such an approach.

Such transactions, in fact, are not uncommon.

But what was new was the size of the transaction. In the years immediately before GM’s deal, disclosed in June 2012, the total amount of pension liabilities annuitized through deals with insurers didn’t exceed $1 billion a year, let alone a single annuity deal topping that, according to Aon Hewitt.

But GM paid Prudential $25.1 billion for its group annuity, which covered benefits paid to about 76,000 salaried employees who retired before Oct. 1, 1997, as well as to employees who retired after Oct. 1, 1997, and before Dec. 1, 2011, and declined the company’s offer of a cash lump-sum instead of a monthly pension benefit.

While no group annuity deal since GM’s has approached the size of the automaker’s transaction with Prudential, there have been several multi-billion dollar group annuity deals since GM's.

For example, just a few months after GM disclosed its annuity purchase, Verizon Communications Inc. announced that it had bought a giant annuity — also from Prudential — to transfer about $7.5 billion in pension benefits promised to 41,000 Verizon participants who retired before Jan. 1, 2010.

And the big deals keep coming. In February, for example, Dallas-based paper goods manufacturer Kimberly-Clark Corp. disclosed that it was buying two group annuity contracts — one from Prudential and the other from Massachusetts Mutual Life Insurance Co. — to transfer about $2.5 billion of pension benefits promised to about 21,000 retirees.

And, just a few months before that, Motorola Solutions Inc., the Schaumburg, Illinois-based electronics company, announced that it was shifting about $3.1 billion in benefits for 30,000 retirees through a group annuity purchase from Prudential.

Experts say employers increasingly do not want to be exposed to the financial uncertainties inherent with a defined benefit plan such as having to funnel more money into it when investment results are subpar or interest rates decline.

“There is the potential to reduce strategic flexibility,” said Scott Kaplan, a Prudential senior vice president and head of pension risk transfer in Woodbridge, New Jersey, referring to the risk associated with maintaining a defined benefit plan.

“These plans have gotten very large and represent a lot of risk,” added Jason Richards, a senior retirement risk management consultant with Towers Watson & Co. in St. Louis.

In addition, shifting pension plan liabilities to an insurer through a group annuity eliminates the overhead, such as the increasing mandatory insurance premiums paid to the federal Pension Benefit Guaranty Corp., said Wayne Daniel, senior vice president and head of U.S. pensions at MetLife Inc. in New York.

“There is an increased cost to maintain the plans,” Mr. Daniel added.

To be sure, shifting pension plan obligations to insurers has a price too, with premiums equal to roughly 5% to 8% of the liabilities being transferred, consultants say.

But that is a price more employers will be willing to pay, insurers say.

“We are very bullish on the future of this market,” said MetLife’s Mr. Daniel.