Sokol said he was resigning to "invest my family's resources in such a way as to create enduring equity value and hopefully an enterprise [that] will provide opportunity for my descendants and funding for my philanthropic interests."
Calls to Sokol at Berkshire Hathaway were not immediately returned.
Sokol explained to Buffett in the letter: "I have no more detailed plan than this because my obligations from Berkshire Hathaway have been my first and only business priority."
Last year, MidAmerican accounted for about 8 percent of Berkshire's total revenue, according to the Journal. Proceeds from the sale of "Pleased but Not Satisfied" went to the MidAmerican Energy Foundation, which supports education, the arts and social services in the communities where MidAmerican Energy Holdings Company" has operations, according to the publisher's website.
Sokol was also chairman and CEO of the private aviation company NetJets Inc., and building material manufacturer Johns Manville.
In a note to investors this morning, Doug Kass, a prominent money manage, said Sokol's resignation will not be a huge blow to Berkshire Hathaway's reputation.
While Sokol is "a loss to Berkshire," "there are several equally qualified replacements for the oracle," said Kass. "Moreover, I have long thought that Sokol's frequent requests to retire from the company suggested it was unlikely that he was going to replace Buffett. ... That said, Mr. Buffett is not going anywhere for now."
ABC News' Bianna Golodryga contributed to this story.