New York (CNN) A rough ride for the stock markets at the end of last year battered Warren Buffett's Berkshire Hathaway: It posted a rare net loss of $25 billion for the fourth quarter.

Troubles at Kraft Heinz also cut deeply into the investing giant's bottom line. In his annual letter to investors posted Saturday, Buffett said the company took a $3 billion write down last year "arising almost entirely from our equity interest in

The loss was driven by big price declines in Berkshire's investment portfolio, includingwhich remains its largest holding.

Buffett also reiterated his criticism of a new accounting rule that he says over-exaggerate Berkshire's performance by requiring the company to account for short-term stock fluctuations. Wild stock swings in December, the letter says, had Berkshire recording "several days with a 'profit' or 'loss' of more than $4 billion."

Overall, the investing giant was $4 billion in the black last year. And Buffett, who is famous for championing long-term performance over quarterly results, urged investors not to forget the forest for the trees.

"Investors who evaluate Berkshire sometimes obsess on the details of our many and diverse businesses -- our economic 'trees', so to speak."

Its overall portfolio was valued at $173 billion at the end of 2018.

Buffett said Berkshire does not see any potential takeover targets in the coming year — but the firm is hungry to make an "elephant-sized acquisition."

"Just writing about the possibility of a huge purchase has caused my pulse rate to soar," Buffett said.

Buybacks

A surge in corporate buybacks have been the target of mounting criticism from Washington. But Buffett sung its praises in his annual letter and said that Berkshire will likely be a "significant repurchaser" of its own shares.

He said he was pleased that many companies that Berkshire has major stakes in are embracing buybacks because they can automatically boost Berkshire's ownership percentage.

Many Democrats and at least one major Republican have expressed concern that US companies aren't investing enough of their windfall from the 2017 tax cut, which slashed the corporate tax rate from 35% to 21%, into job-creating investments.