Securian Financial Group Inc., the insurer formed in 1880, agreed to buy a majority stake in Asset Allocation & Management Co. from a private-equity fund overseen by Stone Point Capital LLC.

The deal will add $16.4 billion in assets under management for other insurers, St. Paul, Minn.-based Securian said in a statement that didn’t disclose terms. AAM will operate as a subsidiary and retain its management, Chicago location and brand.

“The transaction deepens our commitment to the asset-management business and supports our long-term strategy,” said Chris Hilger, who will take over as Securian’s chief executive officer in January. “AAM has a strong reputation in the insurance asset-management market and a diversified base of clients.”

Insurers have been buying asset managers to add fee revenue from third-party clients. Cathay Financial Holding Co. agreed this month to buy Conning Holdings Corp., which focuses on insurance clients. Allied World Assurance Co. and Principal Financial Group Inc. have also invested in money managers.

Securian’s Advantus Capital Management will retain its insurance clients while pursuing growth among retail and institutional investors such as pension funds. AAM will seek new insurance clients.

Securian previously struck deals to expand with the purchase of operations that didn’t fit with the main businesses at some of the world’s largest financial firms. It announced agreements in 2011 to buy two units from Munich Re and a life insurance business from Bank of America Corp.

Stone Point, which counts former Goldman Sachs Group Inc. director Stephen Friedman as chairman, has insurance-related holdings including the largest investment in Enstar Group Ltd.

Goldman Sachs represented Securian, according to the statement. Keefe Bruyette & Wood Inc. was the bank for sellers led by Stone Point. The deal is expected to be completed by Dec. 31, Securian said.