St. Louis–based utility holding company The Laclede Group Inc. said Monday it has agreed to pay about $1 billion to acquire a pair of gas utilities—Missouri Gas Energy and New England Gas Company—from Southern Union Company, an affiliate of Dallas-based pipeline operator Energy Transfer Equity.

The two utilities, which between them serve more than 550,000 customers in Massachusetts and western Missouri, had combined revenues of $517 million during the fiscal year ending September 30. The deal would essentially double the size of Laclede, which currently serves some 630,000 customers in St. Louis and other parts of eastern Missouri through its Laclede Gas Company unit.