Warren Buffett's Berkshire Hathaway Inc said it owned 860.6 million U.S. dollars of Amazon.com Inc shares at the end of March, after the billionaire admitted to underestimating the online retailer and its Chief Executive Jeff Bezos.

The size of the investment was disclosed in a regulatory filing published on Wednesday, which detailed Berkshire's U.S.-listed stocks as of March 31, including investments by Buffett and his portfolio managers Todd Combs and Ted Weschler.

Buffett told CNBC this month that the 483,300 share Amazon investment, which would now be worth 904 million U.S. dollars, 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.

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

These have included Berkshire's 32.1-billion-U.S.-dollar purchase in 2016 of industrial and aircraft parts maker Precision Castparts and a stake in Apple Inc worth close to 48 billion U.S. dollars.

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

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