Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
REVENUES
Commissions									$ 320,114	$ 329,668	$ 336,620
Investment Advisory Fee Revenue									353,671	314,349	320,746
Investment banking									126,211	115,353	78,215
Sweep Interest Income from FDIC-Insured Bank Deposits									117,422	116,052	76,839
Interest									50,723	52,484	48,498
Principal transactions, net									30,094	14,461	23,273
Other									35,144	15,787	36,147
Total revenue	$ 295,881	$ 234,793	$ 250,935	$ 251,770	$ 243,254	$ 237,814	$ 242,556	$ 234,530	1,033,379	958,154	920,338
EXPENSES
Compensation and related expenses									657,714	607,192	602,138
Communications and technology									81,588	74,479	71,978
Occupancy and equipment costs									62,198	61,171	61,164
Clearing and exchange fees									21,962	22,985	23,545
Interest									45,687	46,396	28,354
Other									89,318	101,062	113,607
Total expenses	260,908	228,297	233,544	235,718	227,677	230,660	230,039	224,909	958,467	913,285	900,786
Income (loss) before income taxes from continuing operations	34,973	6,496	17,391	16,052	15,577	7,154	12,517	9,621	74,912	44,869	19,552
Other Income Tax Expense (Benefit), Continuing Operations	9,538	2,547	5,016	4,858	7,316	2,083	3,662	2,916	21,959	15,977	(2,134)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									52,953	28,892	21,686
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax									0	0	2,071
Equity in earnings of subsidiaries									0	0	0
Discontinued Operation, Tax Effect of Discontinued Operation									0	0	941
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0	0	1,130
Net income (loss) attributable to Oppenheimer Holdings Inc.	$ 25,435	$ 3,949	$ 12,375	$ 11,194	$ 8,261	$ 5,071	$ 8,855	$ 6,705	52,953	28,892	22,816
Other comprehensive income									1,596	(1,417)	2,263
Total comprehensive income									54,549	27,475	25,079
Eliminations
REVENUES
Commissions									0	0	0
Investment Advisory Fee Revenue									(4,347)	(4,418)	(4,120)
Investment banking									0	0	(3,000)
Sweep Interest Income from FDIC-Insured Bank Deposits									0	0	0
Interest									(8,290)	(8,310)	(9,639)
Principal transactions, net									(12)	(54)	0
Other									1	(438)	(359)
Total revenue									(12,648)	(13,220)	(17,118)
EXPENSES
Compensation and related expenses									(7)	0	0
Communications and technology									0	0	0
Occupancy and equipment costs									1	(439)	(359)
Clearing and exchange fees									0	0	0
Interest									(8,291)	(8,309)	(9,639)
Other									(4,351)	(4,418)	(7,120)
Total expenses									(12,648)	(13,166)	(17,118)
Income (loss) before income taxes from continuing operations									0	(54)	0
Other Income Tax Expense (Benefit), Continuing Operations									0	0	0
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0	(54)	0
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax											0
Equity in earnings of subsidiaries									(122,769)	(77,285)	(48,588)
Discontinued Operation, Tax Effect of Discontinued Operation											0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent											0
Net income (loss) attributable to Oppenheimer Holdings Inc.									(122,769)	(77,339)	(48,588)
Other comprehensive income									0	0	0
Total comprehensive income									(122,769)	(77,339)	(48,588)
Parent
REVENUES
Commissions									0	0	0
Investment Advisory Fee Revenue									0	0	0
Investment banking									0	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits									0	0	0
Interest									237	66	0
Principal transactions, net									0	0	0
Other									3	0	22
Total revenue									240	66	22
EXPENSES
Compensation and related expenses									1,323	1,548	1,237
Communications and technology									172	163	160
Occupancy and equipment costs									0	0	0
Clearing and exchange fees									0	0	0
Interest									12,319	13,500	13,740
Other									2,814	1,208	4,969
Total expenses									16,628	16,419	20,106
Income (loss) before income taxes from continuing operations									(16,388)	(16,353)	(20,084)
Other Income Tax Expense (Benefit), Continuing Operations									(4,609)	(4,371)	(7,110)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									(11,779)	(11,982)	(12,974)
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax											0
Equity in earnings of subsidiaries									64,732	40,874	35,790
Discontinued Operation, Tax Effect of Discontinued Operation											0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent											0
Net income (loss) attributable to Oppenheimer Holdings Inc.									52,953	28,892	22,816
Other comprehensive income									0	0	0
Total comprehensive income									52,953	28,892	22,816
Guarantor Subsidiaries
REVENUES
Commissions									0	0	0
Investment Advisory Fee Revenue									1,853	1,938	1,752
Investment banking									0	0	0
Sweep Interest Income from FDIC-Insured Bank Deposits									0	0	0
Interest									8,238	8,247	9,589
Principal transactions, net									118	0	17
Other									2	443	361
Total revenue									10,211	10,628	11,719
EXPENSES
Compensation and related expenses									0	0	0
Communications and technology									0	0	0
Occupancy and equipment costs									(1)	0	0
Clearing and exchange fees									0	0	0
Interest									0	0	0
Other									1,366	4,059	1,382
Total expenses									1,365	4,059	1,382
Income (loss) before income taxes from continuing operations									8,846	6,569	10,337
Other Income Tax Expense (Benefit), Continuing Operations									2,151	2,052	(12,655)
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									6,695	4,517	22,992
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax											0
Equity in earnings of subsidiaries									58,037	36,411	12,798
Discontinued Operation, Tax Effect of Discontinued Operation											0
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent											0
Net income (loss) attributable to Oppenheimer Holdings Inc.									64,732	40,928	35,790
Other comprehensive income									0	0	0
Total comprehensive income									64,732	40,928	35,790
Non-Guarantor Subsidiaries
REVENUES
Commissions									320,114	329,668	336,620
Investment Advisory Fee Revenue									356,165	316,829	323,114
Investment banking									126,211	115,353	81,215
Sweep Interest Income from FDIC-Insured Bank Deposits									117,422	116,052	76,839
Interest									50,538	52,481	48,548
Principal transactions, net									29,988	14,515	23,256
Other									35,138	15,782	36,123
Total revenue									1,035,576	960,680	925,715
EXPENSES
Compensation and related expenses									656,398	605,644	600,901
Communications and technology									81,416	74,316	71,818
Occupancy and equipment costs									62,198	61,610	61,523
Clearing and exchange fees									21,962	22,985	23,545
Interest									41,659	41,205	24,253
Other									89,489	100,213	114,376
Total expenses									953,122	905,973	896,416
Income (loss) before income taxes from continuing operations									82,454	54,707	29,299
Other Income Tax Expense (Benefit), Continuing Operations									24,417	18,296	17,631
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									58,037	36,411	11,668
Discontinued Operation, Income (Loss) from Discontinued Operation, before Income Tax											2,071
Equity in earnings of subsidiaries									0	0	0
Discontinued Operation, Tax Effect of Discontinued Operation											941
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent											1,130
Net income (loss) attributable to Oppenheimer Holdings Inc.									58,037	36,411	12,798
Other comprehensive income									1,596	(1,417)	2,263
Total comprehensive income									$ 59,633	$ 34,994	$ 15,061