Magellan Health Services Inc. said in a federal filing Monday that one of its customers will end a contract at the end of 2013.

The customer was not named. The customer's contract brought in $144.5 million in revenue to Magellan during the nine-month period ending Sept. 30.

Magellan, based in Avon, wrote in a filing with the U.S. Securities & Exchange Commission: "The customer has informed the company that, after a competitive evaluation process, it has decided not to renew its contract with the company and to contract with another behavioral health management vendor."

Magellan declined to name the customer or offer any further details about the contract.

Magellan Health is a specialty insurer that sells coverage for behavioral, radiology and pharmaceutical benefits.

Shares of the company closed at $49.00, down $2.75 on Monday afternoon.