Regulatory Requirements - Additional Information (Narrative) (Details)	6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 EUR (€)
Regulatory Capital Requirements [Line Items]
Net capital	$ 3,100,000
Excess Liquid Capital under Hong Kong SFC Rules	2,700,000
Oppenheimer [Member]
Regulatory Capital Requirements [Line Items]
Required percentage of net capital to aggregate customer-related debit items	0.02
Net Capital	$ 251,300,000
Aggregate indebtedness	21.83%
Excess capital	$ 228,300,000
Freedom
Regulatory Capital Requirements [Line Items]
Net Capital	$ 5,000,000.0
Aggregate indebtedness	6.67%
Freedom maintain net capital equal to the greater	$ 100,000
Net capital in excess of minimum required	$ 4,900,000
Oppenheimer Europe Ltd
Regulatory Capital Requirements [Line Items]
Common Equity Tier 1 Ratio	23.36%	23.36%
Common Equity Tier 1 Ratio Required	4.50%	4.50%
Tier 1 Capital Ratio	23.36%	23.36%
Tier 1 Capital Ratio Required	6.00%	6.00%
Total Capital Ratio	31.15%	31.15%
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	$ 387,000