Berkshire Hathaway Assurance Corp., Columbia Insurance Co., General Reinsurance Corp. and Government Employees Insurance Company (GEICO) were other insurance companies affected by the downgrade.
In his annual letter to investors from late February, Buffett acknowledged Berkshire's $7.5 billion investment and derivative losses, but said the U.S. still had its best days ahead of it.
"Though the path has not been smooth, our economic system has worked extraordinarily well over time," Buffett wrote. "It has unleashed human potential as no other system has, and it will continue to do so.
Moody's said it could further downgrade Berkshire if it took on more risk, experienced continued losses in insurance underwriting, investments or derivatives that cause a 20% in shareholder equity or a "material decline" in operating cash flow.