Citigroup Global Markets has agreed to pay $18.3 million to settle charges that it overbilled investment advisory clients and misplaced client contracts, the Securities and Exchange Commission announced Thursday.

The SEC’s order finds that at least 60,000 advisory clients were overcharged approximately $18 million in unauthorized fees because Citigroup failed to confirm the accuracy of billing rates entered into its computer systems in comparison to fee rates outlined in client contracts, billing histories and other documents.

Citigroup also improperly collected fees during time periods when clients suspended their accounts. The billing errors occurred during a 15-year period, the SEC says. The affected clients have already been reimbursed.

“Advisory clients have every expectation that the fees charged by their financial advisor reflect the negotiated rate,” said Andrew M. Calamari, director of the SEC’s New York Regional Office, in a statement. “Citigroup failed to take the necessary precautions to ensure clients were billed in a manner consistent with their advisory agreements.”

The SEC also says Citigroup cannot locate approximately 83,000 advisory accounts opened from 1990 to 2012. Without those missing advisory contracts, Citigroup could not properly validate whether the fee rates negotiated by clients when accounts were opened were the same advisory fee rates being billed to clients over the years.

It is estimated that Citigroup received approximately $3.2 million in excess fees from advisory clients whose contracts were lost, the complaint says.

“It’s a fundamental responsibility of a financial advisor to preserve key account documents such as advisory contracts,” said Sanjay Wadhwa, senior associate director at the SEC’s New York office. “Citigroup failed to safeguard its client contracts, which seriously impeded its ability to determine the proper amount of fees the firm was authorized to charge.”

Citigroup consented to the SEC’s cease-and-desist order and agreed to undertakings related to its fee-billing and books-and-records practices. The firm is censured and must pay $3.2 million in disgorgement of the excess fees collected due to the missing contracts plus $800,000 in interest and a $14.3 million penalty, the SEC says.