Earnings per share - Schedule of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Basic weighted average number of shares outstanding (in shares)	10,642,909	10,465,771
Net dilutive effect of share-based awards, treasury stock method (in shares)		812,168
Diluted weighted average number of shares outstanding (in shares)	10,642,909	11,277,939
Net (loss) income attributable to Oppenheimer Holdings Inc.	$ (20,578)	$ 30,655
(Loss) Earnings per share attributable to Oppenheimer Holdings Inc. - Basic (in dollars per share)	$ (1.93)	$ 2.93
(Loss) Earnings per share attributable to Oppenheimer Holdings Inc. - Diluted (in dollars per share)	$ (1.93)	$ 2.72
Class A Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of anti-dilutive stock options granted (in shares)	983,778	211,250