Comcast Chairman and CEO Brian Roberts called the deal "pro-competitive" and "pro-consumer," and said he's confident it will be approved despite concerns about the two largest cable companies combining.
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"We're going to be able to bring better products, faster Internet, more channels, On-Demand, TV Everywhere, in a national-local platform that's really special," Roberts said in a CNBC "Squawk Box" interview.
The deal was worth $158.82 per share at Wednesday's closing stock prices, a premium of just over 17% to Time Warner Cable's shares.
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Comcast shares fell while Time Warner Cable rose sharply Thursday. At the close the deal's value fell to $43.4 billion and the premium fell to 5.2 percent.
The new company would be by far the largest cable provider in the nation with more than 33 million subscribers. It is expected to face a tough review from the Federal Communications Commission.
"We want them to own more cable, and they're buying Time Warner Cable at what appears to be a very a reasonable price. I do think it gets done," Gabelli Funds portfolio manager Chris Marangi said in a CNBC interview.
Like Roberts, Time Warner Cable Chairman and CEO Robert Marcus said he felt confident the deal would clear regulatory hurdles. He added that the deal maximizes shareholder value.
Roberts said the deal will give the cable and media company a wider distribution of its products, adding that all of Comcast's competitors are national players and this deal puts it on a level playing field.
"One of the things that this transaction allows is about a $1.5 billion worth of synergies. That's mostly because we don't have any overlap in our markets and we're able to reduce some of the overhead, some of the duplication where we both have to produce guides and network operations," he said.
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Time Warner Cable owns cable systems in key areas, including New York City, Southern California, and Texas. Of the top 50 market areas in the country, Time Warner Cable and Comcast overlap in only two — New York City and Kansas City.
"Time Warner Cable having New York and LA—several million subscribers in the two most important cities in the country—this is an unparalleled, unique asset that is never going to come available for sale again," said Richard Greenfield, media and technology analyst at BTIG.
Les Moonves, the CBS chief executive whose company had an ugly battle over fees with Time Warner Cable last summer, said the deal took him by surprise. But he added that a larger Comcast would not necessarily affect what CBS would be paid for its programming.
"At the end of the day, if you have the right content you're always going to have the power," he said in a CNBC interview.
The agreement, first reported by CNBC on Wednesday evening, comes more than eight months after Charter Communications and Liberty Media made their first foray to try to negotiate a deal to acquire Time Warner Cable. At one point sources had suggested Charter and Comcast could team up on a joint bid.
Other suitors had been looking at Time Warner Cable as well.
Charter's offer of roughly $133 a share in cash and stock was rejected by Time Warner Cable as it held out for a price of $160, which it considered a fairer value for the company.
"Sometimes the student in an industry schools the teacher. I think this is a case where Brian Roberts did a major one-upping ... [of] the dean of the cable industry in John Malone," Greenfield said. "This is something John wanted, and Liberty wanted, and Charter wanted very, very badly."
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In an effort to head off concerns about it being too large after the combination, Comcast said it was preparing to divest systems representing 3 million subscribers, though it was not immediately clear where.
Roberts said, "We'd be happy to talk to John [Malone] or anybody else about that opportunity."
(Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC and CNBC.com.)