The size of the investment was disclosed in a regulatory filing detailing Berkshire’s US-listed stocks as of March 31.

Wednesday’s filing includes investments by Buffett and his portfolio managers Todd Combs and Ted Weschler, but does not say who bought and sold what.

Buffett told CNBC this month that the 483,300 share Amazon investment, which would now be worth USD 904 million, was made by Combs or Weschler, and that he had been “an idiot” for not investing in Amazon himself.

The investment tightens the relationship between Buffett and fellow billionaire Bezos, whose companies teamed up last year with JPMorgan Chase & Co to create a venture, Haven, to reduce their employees’ healthcare costs.

Berkshire also invests in JPMorgan, and boosted its holdings in the largest US bank by 19 per cent in the first quarter to 59.5 million shares, worth USD 6.02 billion.

Bezos has also drawn praise from longtime Berkshire Vice Chairman Charlie Munger.

“I don’t mind not having caught Amazon early. The guy is kind of a miracle worker,” Munger said at Berkshire’s annual shareholder meeting on May 4, referring to Bezos. “It’s very peculiar. I give myself a pass on that.”

Combs and Weschler’s initial investments have sometimes heralded major Berkshire commitments directed by Buffett.

These have included Berkshire’s USD 32.1 billion purchase in 2016 of industrial and aircraft parts maker Precision Castparts, and a stake in Apple Inc worth close to USD 48 billion.

Investors monitor Berkshire’s quarterly filings for signs about how where Buffett, Combs and Weschler see value, and stock prices often rise when Berkshire discloses new stakes.

Berkshire also owns more than 90 businesses such as the BNSF railroad and Geico auto insurer.

It often buys stocks when buying whole companies appear too costly. Buffett said on Feb 23 that prices were “sky-high for businesses possessing decent long-term prospects.” The Standard & Poor’s 500 has since risen 2.1 per cent.

Among other portfolio changes, Berkshire reported adding to its stakes in Delta Air Lines Inc, PNC Financial Services Group Inc and software company Red Hat Inc, and paring stakes in oil refiner Phillips 66, Southwest Airlines Co and Wells Fargo & Co.