Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
REVENUES
Commissions	$ 79,409	$ 83,407
Investment Advisory Fee Revenue	73,647	77,548
Investment banking	28,043	28,210
Sweep Interest Income from FDIC-Insured Bank Deposits	33,968	25,297
Interest	12,727	12,227
Principal transactions, net	11,438	2,726
Other	12,538	5,115
Total revenue	251,770	234,530
EXPENSES
Compensation and related expenses	160,355	153,104
Communications and technology	20,086	18,688
Occupancy and equipment costs	15,273	15,428
Clearing and exchange fees	5,332	6,096
Interest	12,986	8,963
Other	21,686	22,630
Total expenses	235,718	224,909
Operating Income (Loss)	16,052	9,621
Income Tax Expense (Benefit)	4,858	2,916
Net Income (Loss) before equity in Earnings of Subsidiaries	11,194	6,705
Equity in earnings of subsidiaries	0	0
Net income attributable to Oppenheimer Holdings Inc.	11,194	6,705
Other comprehensive income	563	(142)
Total comprehensive income	11,757	6,563
Eliminations
REVENUES
Commissions	0	0
Investment Advisory Fee Revenue	(335)	(326)
Investment banking	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0
Interest	(2,073)	(2,080)
Principal transactions, net		(33)
Other	0	(95)
Total revenue	(2,408)	(2,534)
EXPENSES
Compensation and related expenses	0	0
Communications and technology	0	0
Occupancy and equipment costs	0	(95)
Clearing and exchange fees	0	0
Interest	(2,074)	(2,080)
Other	(334)	(359)
Total expenses	(2,408)	(2,534)
Operating Income (Loss)	0	0
Income Tax Expense (Benefit)	0	0
Net Income (Loss) before equity in Earnings of Subsidiaries	0	0
Equity in earnings of subsidiaries	(26,825)	(17,945)
Net income attributable to Oppenheimer Holdings Inc.	(26,831)	(17,945)
Other comprehensive income		0
Total comprehensive income	(26,831)	(17,945)
Guarantor Subsidiaries
REVENUES
Commissions	0	0
Investment Advisory Fee Revenue	0	0
Investment banking	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0
Interest	2,062	2,060
Principal transactions, net	49	0
Other	1	96
Total revenue	2,112	2,156
EXPENSES
Compensation and related expenses	0	0
Communications and technology	0	0
Occupancy and equipment costs	0	0
Clearing and exchange fees		0
Interest	0	0
Other	122	253
Total expenses	122	253
Operating Income (Loss)	1,990	1,903
Income Tax Expense (Benefit)	926	264
Net Income (Loss) before equity in Earnings of Subsidiaries	1,064	1,639
Equity in earnings of subsidiaries	12,883	8,153
Net income attributable to Oppenheimer Holdings Inc.	13,947	9,792
Other comprehensive income	0	0
Total comprehensive income	13,947	9,792
Non-Guarantor Subsidiaries
REVENUES
Commissions	79,409	83,407
Investment Advisory Fee Revenue	73,982	77,874
Investment banking	28,043	28,210
Sweep Interest Income from FDIC-Insured Bank Deposits	33,968	25,297
Interest	12,654	12,247
Principal transactions, net	11,389	2,759
Other	12,537	5,114
Total revenue	251,982	234,908
EXPENSES
Compensation and related expenses	159,910	152,778
Communications and technology	20,047	18,649
Occupancy and equipment costs	15,273	15,523
Clearing and exchange fees	5,332	6,096
Interest	11,685	7,668
Other	21,582	22,349
Total expenses	233,829	223,063
Operating Income (Loss)	18,153	11,845
Income Tax Expense (Benefit)	5,275	3,692
Net Income (Loss) before equity in Earnings of Subsidiaries	12,878	8,153
Equity in earnings of subsidiaries	0	0
Net income attributable to Oppenheimer Holdings Inc.	12,884	8,153
Other comprehensive income	563	(142)
Total comprehensive income	13,447	8,011
Parent
REVENUES
Commissions	0	0
Investment Advisory Fee Revenue	0	0
Investment banking	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits	0	0
Interest	84	0
Principal transactions, net	0	0
Other	0	0
Total revenue	84	0
EXPENSES
Compensation and related expenses	445	326
Communications and technology	39	39
Occupancy and equipment costs	0	0
Clearing and exchange fees	0	0
Interest	3,375	3,375
Other	316	387
Total expenses	4,175	4,127
Operating Income (Loss)	(4,091)	(4,127)
Income Tax Expense (Benefit)	(1,343)	(1,040)
Net Income (Loss) before equity in Earnings of Subsidiaries	(2,748)	(3,087)
Equity in earnings of subsidiaries	13,942	9,792
Net income attributable to Oppenheimer Holdings Inc.	11,194	6,705
Other comprehensive income	0	0
Total comprehensive income	$ 11,194	$ 6,705