5:53 p.m. | Updated

Bloomberg L.P., the financial data giant, said on Thursday that it had agreed to buy the Bureau of National Affairs for $990 million, its biggest acquisition.

The bureau’s lineup of news services, which provides legal, tax, regulatory and environmental information to professionals in those fields, is expected to bolster Bloomberg’s existing financial platform and help its Bloomberg Law division compete with legal information services like Westlaw.

The deal also represents the end of an era at the Bureau of National Affairs. Founded in 1926 by David Lawrence, a newspaper reporter who later started a precursor to U.S. News and World Report, the company has been entirely owned by current or former employees since 1947. Bloomberg’s acquisition will cash out B.N.A.’s employees in a deal that is expected to close by the end of the year.

“B.N.A.’s employees have built a superior franchise and we are enthusiastic about a Bloomberg-B.N.A. combination that will deliver more premium content to our professional audiences,” Daniel L. Doctoroff, Bloomberg’s president and chief executive, said in a statement. “B.N.A. research and analysis will make Bloomberg’s products even more valuable, and B.N.A. would benefit from our data and technology expertise.”

Under the terms of the deal, Bloomberg will pay $39.50 a share in cash through a tender offer, after which B.N.A. will become a stand-alone subsidiary of the company. The tender offer will begin on Sept. 8.

Bloomberg L.P. is controlled by Mayor Michael R. Bloomberg of New York, who started the company 30 years ago.

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