Separately, S&P said it was nervous about the exchange's growing profile in credit default swaps because the products are "outside the clearinghouse's historical expertise".
The company's clearing volumes for over-the-counter interest rate swaps and credit default swaps rose significantly in the fourth quarter. CME said it was "pleased with the growth".
The ratings downgrade recognizes that off-exchange business transfers some counterparty risk to CME, said Craig Pirrong, a professor and a director for the Global Energy Management Institute at the University of Houston.
"The whole narrative about the reason we should have clearing is that it essentially makes counterparty risk go away," he said. "It doesn't. It essentially moves it around, and some of that risk has moved to the CME. That's reflected in the lower credit rating."
Yet, MF Global's collapse was likely a bigger factor in CME's downgrade than the expansion of off-exchange business, said Michael Greenberger, a law professor at the University of Maryland. He was the former director of trading and markets for the Commodity Futures Trading Commission, the main U.S. futures industry regulator.
"I think what the credit agency is saying is if you can't get your old business right, what is going to happen to you when you start handling business that is seven times that which you already have," said Greenberger, referring to CME's growth in over-the-counter business.
CME last week said it earned $745.9 million, or $11.25 per share, in the fourth quarter, up from $196.2 million, or $2.93 per share, a year earlier.
Total trading volume fell 2 percent from a year earlier, although average revenue per contract rose 4 percent from the third quarter to 81.1 cents. The firm cited increased volume in higher-priced commodity contracts.