Looks like they entered into total return swaps to buyback over 5% of the outstanding shares during q4:
"Throughout much of last year, I made public statements to the effect that our belief was that Fairfax shares were trading in the market at a ridiculously cheap price. Following our value investing philosophy, since the latter part of 2020 we have purchased total return swaps with respect to 1,407,864 subordinate voting shares of Fairfax with a total market value at the time of those agreements of $484.9 million ($344.45 (Cdn$443.93) per share)," said Prem Watsa, Chairman and Chief Executive Officer.
Also directly bought back several hundred thousand shares:
During 2020 the company purchased 457,603 subordinate voting shares for treasury and 343,871 for cancellation at an aggregate cost of $238.8 million.