Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
REVENUES
Commissions	$ 101,636	$ 80,896	$ 204,885	$ 160,305
Investment Advisory Fee Revenue	75,981	80,707	162,145	154,354
Investment banking	46,186	32,006	71,914	60,049
Sweep Interest Income from FDIC-Insured Bank Deposits	7,122	31,830	25,948	65,798
Interest	6,220	13,550	17,110	26,277
Principal transactions, net	12,064	3,045	11,196	14,483
Other	15,521	8,901	6,302	21,439
Total revenue	264,730	250,935	499,500	502,705
EXPENSES
Compensation and related expenses	179,594	155,783	337,270	316,138
Communications and technology	21,324	20,499	41,215	40,585
Occupancy and equipment costs	15,334	15,573	31,412	30,846
Clearing and exchange fees	6,191	5,678	11,850	11,010
Interest	2,890	13,192	9,440	26,178
Other	16,133	22,819	34,826	44,505
Total expenses	241,466	233,544	466,013	469,262
Operating Income (Loss)	23,264	17,391	33,487	33,443
Income Tax Expense (Benefit)	5,615	5,016	8,020	9,874
Net Income (Loss) before equity in Earnings of Subsidiaries	17,649	12,375	25,467	23,569
Equity in earnings of subsidiaries	0	0	0	0
Net income attributable to Oppenheimer Holdings Inc.	17,649	12,375	25,467	23,569
Other comprehensive income	480	361	(57)	924
Comprehensive income	18,129	12,736	25,410	24,493
Eliminations
REVENUES
Commissions	0	0	0	0
Investment Advisory Fee Revenue	(349)	(360)	(703)	(695)
Investment banking	0	0	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0	0	0
Interest	(2,079)	(2,074)	(4,146)	(4,147)
Principal transactions, net	0	0	0	(12)
Other	0	1	(4)	1
Total revenue	(2,428)	(2,433)	(4,853)	(4,853)
EXPENSES
Compensation and related expenses	0	1	0	(5)
Communications and technology	0	0	0	0
Occupancy and equipment costs	0	0	0	0
Clearing and exchange fees	0	0	0	0
Interest	(2,078)	(2,073)	(4,146)	(4,147)
Other	(350)	(361)	(707)	(701)
Total expenses	(2,428)	(2,433)	(4,853)
Operating Income (Loss)	0	0	0
Income Tax Expense (Benefit)	0	0	0
Net Income (Loss) before equity in Earnings of Subsidiaries	0	0	0
Equity in earnings of subsidiaries	(38,200)	(29,191)	(56,925)
Net income attributable to Oppenheimer Holdings Inc.	(38,200)	(29,191)	(56,925)
Other comprehensive income	0	0	0
Comprehensive income	(38,200)	(29,191)	(56,925)
Parent [Member]
REVENUES
Commissions	0	0	0	0
Investment Advisory Fee Revenue	0	0	0	0
Investment banking	0	0	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0	0	0
Interest	1	91	2	175
Principal transactions, net	0	0	0	0
Other	9	0	95	0
Total revenue	10	91	97	175
EXPENSES
Compensation and related expenses	306	380	620	825
Communications and technology	43	52	83	91
Occupancy and equipment costs	0	0	0	0
Clearing and exchange fees	0	0	0	0
Interest	2,506	3,375	5,036	6,750
Other	219	234	452	550
Total expenses	3,074	4,041	6,191	8,216
Operating Income (Loss)	(3,064)	(3,950)	(6,094)	(8,041)
Income Tax Expense (Benefit)	(826)	(995)	(1,566)	(2,338)
Net Income (Loss) before equity in Earnings of Subsidiaries	(2,238)	(2,955)	(4,528)	(5,703)
Equity in earnings of subsidiaries	19,887	15,330	29,995	29,272
Net income attributable to Oppenheimer Holdings Inc.	17,649	12,375	25,467	23,569
Other comprehensive income	0	0	0	0
Comprehensive income	17,649	12,375	25,467	23,569
Guarantor Subsidiaries
REVENUES
Commissions	0	0	0	0
Investment Advisory Fee Revenue	0	0	0	0
Investment banking	0	0	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0	0	0
Interest	2,058	2,060	4,116	4,122
Principal transactions, net	65	40	49	101
Other	0	1	0	2
Total revenue	2,123	2,101	4,165	4,225
EXPENSES
Compensation and related expenses	0	0	0	0
Communications and technology	0	0	0	0
Occupancy and equipment costs	0	0	0	0
Clearing and exchange fees	0	0	0	0
Interest	0	0	0	0
Other	3	122	5	244
Total expenses	3	122	5	244
Operating Income (Loss)	2,120	1,979	4,160	3,981
Income Tax Expense (Benefit)	546	510	1,095	1,436
Net Income (Loss) before equity in Earnings of Subsidiaries	1,574	1,469	3,065	2,545
Equity in earnings of subsidiaries	18,313	13,861	26,930	26,726
Net income attributable to Oppenheimer Holdings Inc.	19,887	15,330	29,995	29,271
Other comprehensive income	0	0	0	0
Comprehensive income	19,887	15,330	29,995	29,271
Non-Guarantor Subsidiaries
REVENUES
Commissions	101,636	80,896	204,885	160,305
Investment Advisory Fee Revenue	76,330	81,067	162,848	155,049
Investment banking	46,186	32,006	71,914	60,049
Sweep Interest Income from FDIC-Insured Bank Deposits	7,122	31,830	25,948	65,798
Interest	6,240	13,473	17,138	26,127
Principal transactions, net	11,999	3,005	11,147	14,394
Other	15,512	8,899	6,211	21,436
Total revenue	265,025	251,176	500,091	503,158
EXPENSES
Compensation and related expenses	179,288	155,402	336,650	315,318
Communications and technology	21,281	20,447	41,132	40,494
Occupancy and equipment costs	15,334	15,573	31,412	30,846
Clearing and exchange fees	6,191	5,678	11,850	11,010
Interest	2,462	11,890	8,550	23,575
Other	16,261	22,824	35,076	44,412
Total expenses	240,817	231,814	464,670	465,655
Operating Income (Loss)	24,208	19,362	35,421	37,503
Income Tax Expense (Benefit)	5,895	5,501	8,491	10,776
Net Income (Loss) before equity in Earnings of Subsidiaries	18,313	13,861	26,930	26,727
Equity in earnings of subsidiaries	0	0	0	0
Net income attributable to Oppenheimer Holdings Inc.	18,313	13,861	26,930	26,727
Other comprehensive income	480	361	(57)	924
Comprehensive income	$ 18,793	$ 14,222	$ 26,873	27,651
Eliminations
EXPENSES
Total expenses				(4,853)
Operating Income (Loss)				0
Income Tax Expense (Benefit)				0
Net Income (Loss) before equity in Earnings of Subsidiaries				0
Equity in earnings of subsidiaries				(55,998)
Net income attributable to Oppenheimer Holdings Inc.				(55,998)
Other comprehensive income				0
Comprehensive income				$ (55,998)