NEW YORK—Warren Buffett’s Berkshire Hathaway Inc. on Monday struck an agreement to buy insurance company Alleghany Corp. for $11.6 billion, only weeks after the 91-year-old billionaire bemoaned a lack of good investment opportunities. Alleghany, the owner of reinsurer Transatlantic Holdings Inc., would expand Berkshire’s large portfolio of insurers, which includes auto insurer Geico, reinsurer General Re, and a unit that insures against major catastrophes and unusual risks. “Berkshire will be the perfect permanent home for Alleghany, a company that I have closely observed for 60 years,” Buffett, who has run Berkshire since 1965, said in a statement. The acquisition, one of the five largest in Berkshire’s history, would reunite Buffett with Joseph Brandon, who led General Re from 2001 to 2008 and became Alleghany’s chief executive in December. It would also end Buffett’s six-year drought of large acquisitions and help him to deploy some of the $146.7 billion of cash …