Income Taxes - Additional Information (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Income Taxes [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	$ 231,000
Unrecognized Tax Benefit, Reversal of Income Tax Interest Payable		$ 104,000
Profit (loss) before income taxes for foreign operations	$ (8,500,000)	$ (965,000)	$ 732,000
Effective Income Tax Rate Reconciliation, Percent	(10.80%)	56.00%	(16.70%)
Effective Income Tax Reconciliation, impact of the TCJA Tax Reform	$ 9,013,000	$ 0	$ 0
Impact of the TCJA Tax Reform, adjustment related to re-measurement of deferred tax liabilities	29,000,000
Impact of the TCJA Tax Reform, adjustment related to re-measurement of deferred tax assets	19,600,000
Impact of the TCJA Tax Reform, Other Adjustments	400,000
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Amount	11,200,000
Unrecognized deferred tax liability related to earnings of foreign subsidiaries	3,000,000
Valuation allowance	$ 1,350,000	1,280,000
Goodwill amortized period	15 years
Unrecognized tax benefits	$ 1,079,000	1,088,000	2,490,000	$ 1,583,000
Unrecognized tax benefit that would impact tax rate	853,000,000,000	710,000
Unrecognized Tax Benefits, Reduction Resulting from Lapse of Applicable Statute of Limitations	9,000	652,000	0
Income tax-related interest payable	232,000	1,000
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$ 0	$ 848,000	$ 0
State and Local Jurisdiction | New York State | Minimum [Member]
Schedule Of Income Taxes [Line Items]
Income tax examination year	2011
State and Local Jurisdiction | New York State | Maximum [Member]
Schedule Of Income Taxes [Line Items]
Income tax examination year	2014
State and Local Jurisdiction | New York City | Minimum [Member]
Schedule Of Income Taxes [Line Items]
Income tax examination year	2009
State and Local Jurisdiction | New York City | Maximum [Member]
Schedule Of Income Taxes [Line Items]
Income tax examination year	2012
Oppenheimer Israel (OPCO) Ltd.
Schedule Of Income Taxes [Line Items]
Net operating loss carryforward	$ 3,100,000
Oppenheimer Europe Ltd
Schedule Of Income Taxes [Line Items]
Net operating loss carryforward	$ 541,000