Cablevision has agreed to be acquired by Altice Group, a European telecom giant, the companies said early Thursday.

Altice will pay $34.90 for each share of Bethpage-based Cablevision Systems Corp. The combined entity will be the fourth largest cable operation in the U.S., Altice said.

The deal values Cablevision at $17.7 billion, including assumed debt, Altice officials said in a conference call Thursday morning.

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Bethpage-based Cablevision Systems Corp. was founded in 1973 by Charles F. Dolan, a cable television pioneer, who is chairman. His son, James L. Dolan, is chief executive. The Dolan family owns a controlling interest in Cablevision, according to regulatory filings.

Patrick Drahi, founder and president of Altice, said in a statement, "As a family business we are proud to be entrusted by the Dolan family with the ownership of Cablevision and look forward to continuing the pioneering path they have paved for us."

James Dolan said in a statement, "the Dolan family has been honored to help shepherd our customers and employees through the most extraordinary communications revolution in modern history." He added, "the time is right for new ownership of Cablevision and its considerable assets. We believe that Patrick Drahi and Altice will be truly worthy successors, and we look forward to doing all we can to affect this transition for our customers and employees. We expect that Cablevision will be in excellent hands."

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Altice said the transaction will be financed with $14.5 billion of new and existing debt at Cablevision, cash on hand at Cablevision, and $3.3 billion of cash from Altice.

At midmorning Thursday, Cablevision shares were trading at $32.92, slightly below the purchase price and up more than 15 percent from Wednesday's closing price.

Cablevision started as a cable television operator serving 1,500 customers on Long Island. It is one of the five largest cable television companies in the United States, according to Bloomberg News.

In addition to cable TV, it offers high-speed Internet access and digital phone service and owns media properties including Newsday, News 12 Long Island and amNewYork. The company recently has expanded into Wi-Fi-only phone service with its Freewheel phone.

Altice executives said on the conference call Thursday morning that they want to keep Cablevision's media assets.

Cablevision has 3.1 million customers, and had 13,656 employees at the end of 2014, according to Bloomberg.

Cablevision is one of only three Long Island companies listed in the Standard & Poor's 500 index. The other two are Henry Schein Inc., a Melville medical supply company, and Kimco, a real estate investment trust based in New Hyde Park.

Drahi, a French entrepreneur, has previously expressed interest in expanding Altice's presence in the U.S. cable industry.

In May, Altice got a foothold in the U.S. market by buying 70 percent of cable group Suddenlink Communications for $9.1 billion.

Suddenlink, based in St. Louis, has about 1.5 million subscribers.

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Drahi's strategy is to stitch together U.S. cable assets and add mobile phone service to TV, telephone and Internet service, forming a "quad play," Thomas Eagan, an analyst at Manhattan research and brokerage firm Telsey Advisory Group, said in July.

Dexter Goei, CEO of Altice, said, "The strategy of Altice in the large and highly strategic U.S. market is reinforced with the acquisition of Cablevision."

Altice Group is headquartered in the Netherlands. Founded in 2002, it has 9,363 employees, according to Bloomberg.

Altice is a multinational cable and telecommunications company with operations in Western Europe -- France, Belgium, Luxembourg, Portugal and Switzerland -- Israel, the French Caribbean, the Dominican Republic and regions of the Indian Ocean.

The deal with Altice won't affect other companies controlled by the Dolan family, including the sports and entertainment company Madison Square Garden Co. and cable channel company AMC Networks.

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