Earnings Per Share - Summary of Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Basic weighted average number of shares outstanding										13,248,876	13,246,423	13,368,768
Net dilutive effect of share-based awards, treasury method	[1]									812,493	426,938	0
Diluted weighted average number of shares outstanding										14,061,369	13,673,361	13,368,768
Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		$ 8,267	$ 5,061	$ 8,839	$ 6,709	$ 21,168	$ 7,403	$ (1,363)	$ (5,338)	$ 28,876	$ 21,870	$ (9,630)
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest		0	0	0	0	29	461	53	587	0	1,130	10,121
Net income		8,267	5,061	8,839	6,709	21,197	7,864	(1,310)	(4,751)	28,876	23,000	491
Net income attributable to non-controlling interest, net of tax		6	(10)	(16)	4	4	75	9	96	(16)	184	1,652
Net income (loss) attributable to Oppenheimer Holdings Inc.		$ 8,261	$ 5,071	$ 8,855	$ 6,705	$ 21,193	$ 7,789	$ (1,319)	$ (4,847)	$ 28,892	$ 22,816	$ (1,161)
Income (Loss) from Continuing Operations, Per Basic Share		$ 0.62	$ 0.38	$ 0.67	$ 0.51	$ 1.61	$ 0.56	$ (0.10)	$ (0.40)	$ 2.18	$ 1.65	$ (0.72)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share		0.00	0.00	0.00	0.00	0.00	0.03	0.00	0.04	0.00	0.07	0.63
Net income (loss) per share		0.62	0.38	0.67	0.51	2	1	0	0	2.18	1.72	(0.09)
Income (Loss) from Continuing Operations, Per Diluted Share		0.59	0.36	0.63	0.48	1.54	0.54	(0.10)	(0.40)	2.05	1.60	(0.72)
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share		0.00	0.00	0.00	0.00	0.00	0.03	0.00	0.04	0.00	0.07	0.63
Diluted earnings per share		$ 0.59	$ 0.36	$ 0.63	$ 0.48	$ 2	$ 1	$ 0	$ 0	$ 2.05	$ 1.67	$ (0.09)

[1] For the year ended December 31, 2018, the diluted net income per share computation does not include the anti-dilutive effect of 4,050 shares of Class A Stock granted under share-based compensation arrangements (10,592 and 1,237,134 shares for the years ended December 31, 2017 and 2016, respectively).(2)Represents net income from discontinued operations less net income attributable to non-controlling interest, net of tax divided by weighted average number of shares outstanding.