The month ended with bond yields slightly lower than the end of August, with the Mercer Yield Curve discount rate for mature pension plans falling from 4.63 percent to 4.58 percent, but still up 87 basis points year-to-date. Mercer estimated that 20 percent of plans sponsored by S&P 1500 companies are now more than 100 percent funded, up dramatically from only 4 percent at the end of 2012.

"It's been a long road – nearly five years – for plan sponsors to get back over a 90 percent funded status," said Jonathan Barry, a partner in Mercer's retirement business. "Furthermore, these plans have recovered over $500 billion from the deficit level in just over a year, from a deficit high-water mark of $689 billion at the end of July 2012.  This demonstrates the level of volatility to which these plans are potentially exposed."

Mercer estimates the aggregate funded status position of plans operated by S&P 1500 companies on a monthly basis. 

Public pension plans have not fared as well because of how public pension assets and liabilities are calculated. One big problem is that large government plans have been calculating their assets based on higher returns of 7 to 8 percent a year, when the actual return is much lower than that.

According to the latest Milliman Public Pension Funding Study, the 100 largest U.S. public pension plans reported assets of $2.7 trillion and accrued liabilities of $3.6 trillion in September, for an aggregate funded ratio of 75.1 percent.