The funded status of the 100 largest corporate defined benefit pension plans improved by a whopping $388 billion in the past year, and $23 billion of that improvement came in July alone.

That's according to the latest report from the Milliman 100 Pension Funding Index, which is based on an annual study of the 100 largest defined benefit pension plans sponsored by U.S. public companies. It found that the deficit dropped to $158 billion from $182 billion thanks to an investment gain of more than 2 percent in July. The PFI funded ratio rose to 89.7 percent, up from 88.2 percent at the end of June.

Pension liabilities increased slightly from $1.54 trillion from $1.538 trillion at the end of June. The change was based on a negligible 1 basis point decrease in the monthly discount rate from 4.74 percent in June to 4.73 percent in July.