Billionaire investor Bill Ackman said today that Pershing Square is increasing its stake in Howard Hughes Holdings by $1 billion and that the real estate developer plans to buy back shares for an additional $500 million.

The deal would increase Pershing Square’s stake in Howard Hughes to somewhere between 61% and 69%, depending on how many investors agree to be bought out, from the 38% it currently holds.

Shares of Howard Hughes, one of Pershing’s longest-held investments, rose 13% to trade at $78.25 before the opening bell.

Through this deal, the real estate company could become a “modern-day Berkshire Hathaway that would acquire controlling interests in operating companies,” Ackman said in a letter to the board of Howard Hughes.

Howard Hughes, spun off from real estate investment trust General Growth Properties in 2010, owns and manages commercial, residential and mixed-use real estate in the United States. It had a market value of $3.6 billion, according to data compiled by LSEG.

Under the proposed deal, a Pershing Square unit will buy 11.8 million shares for $1 billion from non-Pershing Square affiliate shareholders of Howard Hughes, while the real estate developer will commence a $500 million share repurchase at $85 per share for up to 5.9 million shares.

“We, like other long-term shareholders and this board, have been displeased with the company’s stock price performance,” Ackman said in the letter.

Howard Hughes did not immediately respond to a Reuters request for comment.

Howard Hughes’s shareholders can elect to receive the entire payment in cash or “roll over” all or a portion of their shares into the post-merger company, Ackman said in a letter.

Ackman has been personally involved with Howard Hughes for a decade and stepped down from the company’s board in April after having served as its chairman since 2010.