Quarterly Information - Schedule of Quarterly Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 422,908	$ 276,259	$ 264,730	$ 234,770	$ 295,881	$ 234,793	$ 250,935	$ 251,770	$ 1,198,667	$ 1,033,379	$ 958,154
Operating Expenses	309,113	254,541	241,466	224,547	260,908	228,297	233,544	235,718	1,029,667	958,467	913,285
Operating Income (Loss)	113,795	21,718	23,264	10,223	34,973	6,496	17,391	16,052	169,000	74,912	44,869
Income Tax Expense (Benefit)	31,915	6,079	5,615	2,405	9,538	2,547	5,016	4,858	46,014	21,959	15,977
Net income (loss) attributable to Oppenheimer Holdings Inc.	$ 81,880	$ 15,639	$ 17,649	$ 7,818	$ 25,435	$ 3,949	$ 12,375	$ 11,194	$ 122,986	$ 52,953	$ 28,892
Earnings Per Share, Basic	$ 6.56	$ 1.25	$ 1.40	$ 0.61	$ 1.99	$ 0.31	$ 0.95	$ 0.86	$ 9.73	$ 4.10	$ 2.18
Earnings Per Share, Diluted	6.17	1.19	1.34	0.58	1.83	0.29	0.89	0.81	9.30	3.82	$ 2.05
Dividends paid per share	1.12	0.12	0.12	0.12	0.12	0.12	0.11	0.11	1.48	0.46
Market price of Class A Stock
High	33.21	26.11	24.42	28.38	29.25	31.30	27.42	28.73	33.21	31.30
Low	$ 22.29	$ 20.33	$ 17.20	$ 14.88	$ 27.09	$ 26.42	$ 24.34	$ 25.25	$ 14.88	$ 24.34