Regulatory Requirements - Additional Information (Narrative) (Details)	9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2020 EUR (€)
Regulatory Capital Requirements [Line Items]
Net capital	$ 3,500,000
Excess Liquid Capital under Hong Kong SFC Rules	3,100,000
Oppenheimer [Member]
Regulatory Capital Requirements [Line Items]
Required percentage of net capital to aggregate customer-related debit items	0.02
Net Capital	$ 268,700,000
Aggregate indebtedness	20.83%
Excess capital	$ 242,900,000
Freedom
Regulatory Capital Requirements [Line Items]
Net Capital	$ 4,900,000
Aggregate indebtedness	6.67%
Freedom maintain net capital equal to the greater	$ 100,000
Net capital in excess of minimum required	$ 4,800,000
Oppenheimer Europe Ltd
Regulatory Capital Requirements [Line Items]
Common Equity Tier 1 Ratio	0.2087	0.2087
Common Equity Tier 1 Ratio Required	4.50%	4.50%
Tier 1 Capital Ratio	20.87%	20.87%
Tier 1 Capital Ratio Required	6.00%	6.00%
Total Capital Ratio	27.83%	27.83%
Total Capital Ratio Required	8.00%	8.00%
Regulatory capital required to be maintained | €		€ 730,000
Oppenheimer Investments Asia Ltd.
Regulatory Capital Requirements [Line Items]
Regulatory capital required to be maintained	$ 387,089