In his annual letter to shareholders this year, Buffett praised Sokol for engineering a turnaround at NetJets, a business where he had no prior experience, and for his accomplishments at MidAmerican. A year earlier, he called Sokol an enormously talented builder and operator, and in 2009 he proclaimed that Sokol would run any business with which he was associated in a first-class manner. The issue of succession is crucial for Berkshire Hathaway because Buffett personifies the company. He built Berkshire up from a small insurance company to one of the largest insurers and conglomerates in the United States over decades. Berkshire did not release Sokol's March 28 letter of resignation, though Buffett said that in the letter Sokol had mentioned his desire to pursue philanthropic efforts.