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Regulatory Requirements - Additional Information (Narrative) (Details)
1 Months Ended 6 Months Ended
Dec. 31, 2017
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2017
EUR (€)
Oppenheimer [Member]      
Regulatory Capital Requirements [Line Items]      
Required percentage of net capital to aggregate customer-related debit items   $ 0.02  
Net Capital as Reported in Entity's Part II Unaudited FOCUS Report   $ 161,200,000  
Aggregate indebtedness   15.77%  
Excess capital   $ 140,800,000  
Freedom      
Regulatory Capital Requirements [Line Items]      
Net Capital as Reported in Entity's Part II Unaudited FOCUS Report   $ 5,400,000  
Aggregate indebtedness   6.67%  
Freedom maintain net capital equal to the greater   $ 100,000  
Net capital in excess of minimum required   $ 5,300,000.0  
Oppenheimer Europe Ltd      
Regulatory Capital Requirements [Line Items]      
Common Equity Tier 1 Ratio   18.20%  
Common Equity Tier 1 Ratio Required   4.50%  
Tier 1 Capital Ratio   18.20%  
Tier 1 Capital Ratio Required   6.00%  
Total Capital Ratio   18.76%  
Total Capital Ratio Required   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  
Net capital   2,000,000  
Excess Liquid Capital under Hong Kong SFC Rules   $ 1,600,000  
Oppenheimer Europe Ltd      
Regulatory Capital Requirements [Line Items]      
Contributions to subsidiary $ 7,000,000