Employee compensation plans - Narrative (Details)				12 Months Ended
	Jan. 29, 2025 shares	Jan. 05, 2024 shares	Dec. 15, 2021 USD ($)	Dec. 31, 2024 USD ($) plan shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Mar. 01, 2024 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of employee compensation plans | plan				2
Stock options outstanding (in shares) | shares				0	2,447
Maximum annual contribution under Defined Contribution Plan				$ 23,000	$ 22,500	$ 20,500
Employer contribution to Defined Contribution Plan				4,900,000	4,400,000	4,300,000
Deferred bonus amounts							$ 13,400,000
Minimum bonus and salary amount			$ 200,000
Vesting period							3 years
Bonus deferral credit, minimum salary and bonus amount			$ 500,000
Deferral of salary and bonus (as a percent)			50.00%
Elective deferral credit period one			5 years
Elective deferral credit period two			10 years
Elective deferral credit, amount vested (as a percent)			100.00%
Matching credit (as a percent)			10.00%
Deferred compensation plans cost				$ 38,700,000	33,600,000	5,300,000
Class A Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available under the share-based compensation plans (in shares) | shares				999,000				1,000,000
Restricted Stock | Class A Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate intrinsic value of non-option awards				$ 92,000,000.0
Compensation expense relating to the share-based awards				12,900,000	$ 13,100,000	11,500,000
Unrecognized compensation cost related to unvested share-based compensation				$ 20,300,000
Weighted Average Remaining Contractual Life				1 year 7 months 6 days	1 year 7 months 6 days
Non-option awards granted (in shares) | shares				330,140
Vested (in shares) | shares				463,175
Non-option equity instruments, unvested (in shares) | shares				1,434,968	1,589,414
Restricted Stock | Class A Stock | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Non-option awards granted (in shares) | shares	203,375
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense relating to the share-based awards				$ 2,550,000	$ 8,195,000	15,702,000
Stock Appreciation Rights (SARs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period		5 years		5 years
Aggregate intrinsic value of non-option awards				$ 65,300,000
Compensation expense relating to the share-based awards				$ 32,600,000	3,900,000	4,400,000
Weighted Average Remaining Contractual Life				3 years
Non-option awards granted (in shares) | shares		654,280
Non-option equity instruments, unvested (in shares) | shares				2,642,736
Liability related to the non-option equity awards				$ 46,900,000
Total unrecognized compensation cost				$ 27,500,000
Minimum | Restricted Stock | Class A Stock | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	3 years
Maximum | Restricted Stock | Class A Stock | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period	5 years
Third Anniversary | Restricted Stock | Class A Stock | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-option awards granted (in shares) | shares	105,675
Fifth Anniversary | Restricted Stock | Class A Stock | Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares) | shares	97,700
Oppenheimer Holdings 2014 Incentive Plan (OIP) | Minimum | Class A Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				3 years
Oppenheimer Holdings 2014 Incentive Plan (OIP) | Maximum | Class A Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period				5 years
Executive Deferred Compensation Plan and Deferred Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred bonus amounts				$ 12,100,000	$ 10,100,000	$ 11,100,000
Vesting period of deferred bonus				5 years
Executive Deferred Compensation Plan, Deferred Incentive Plan And Capital Markets Deferred Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Company's deferred compensation liability				$ 77,800,000
CIBC Deferred Compensation Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Company's deferred compensation liability				$ 22,400,000