Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 83,073	$ 90,675	$ 42,715	$ 48,154
Deposits with clearing organizations	55,069	67,678
Receivable from brokers, dealers and clearing organizations	180,221	166,493
Receivable from customers, net of allowance for credit losses of $898 ($886 in 2018)	753,625	720,777
Income tax receivable	0	1,014
Securities purchased under agreements to resell	589	290
Securities owned, at fair value	911,468	837,584
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,548 and $6,841 respectively ($25,109 and $6,800, respectively, in 2018)	43,921	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	30,728	28,988
Operating Lease, Right-of-Use Asset	171,224	0
Subordinated loan receivable	0	0
Intangible assets	32,100	32,100
Goodwill	137,889	137,889
Other assets	127,657	112,768
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,527,564	2,240,314
Liabilities
Drafts payable	13,533	16,348
Bank call loans	0	15,000
Payable to brokers, dealers and clearing organizations	634,333	289,207
Payable to customers	334,376	336,616
Securities sold under agreements to repurchase	268,621	484,218
Securities sold but not yet purchased, at fair value	137,286	85,446
Accrued compensation	106,869	167,348
Accounts payable and other liabilities	48,862	87,630
Operating Lease, Liability	212,970	0
Income tax payable	1,162	0
Senior secured notes, net of debt issuance costs of $840 ($904 in 2018)	199,160	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	15,905	14,083
Intercompany payables	0	0
Total liabilities	1,973,077	1,694,992
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	554,487	545,322
Total liabilities and stockholders' equity	2,527,564	2,240,314
Allowance for credit losses	898	886
Amounts pledged	390,832	517,951
Notes Receivable, Net Accumulated Amortization	24,548	25,109
Notes Receivable, Net Allowance for Uncollectibles	6,841	6,800
Net accumulated depreciation	90,974	89,182
Unamortized debt issuance expense	840	904
Operating Lease Right-of-use Asset Accumulated Amortization	6,797	0
Eliminations
ASSETS
Cash and cash equivalents	0	0	0	0
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers, net of allowance for credit losses of $898 ($886 in 2018)	0	0
Income tax receivable	(68,800)	(67,508)
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,548 and $6,841 respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Operating Lease, Right-of-Use Asset	0
Subordinated loan receivable	(112,558)	(112,558)
Intangible assets	0	0
Goodwill	0	0
Other assets	0	0
Deferred tax assets	(17,236)	(18,950)
Investment in subsidiaries	(1,215,660)	(1,208,541)
Intercompany receivables	(26,330)	(26,330)
Total assets	(1,440,584)	(1,433,887)
Liabilities
Drafts payable	0	0
Bank call loans	0	0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	(8)	(9)
Operating Lease, Liability	0
Income tax payable	(68,799)	(67,507)
Senior secured notes, net of debt issuance costs of $840 ($904 in 2018)	0	0
Subordinated indebtedness	(112,558)	(112,558)
Deferred tax liabilities, net	(17,230)	(18,946)
Intercompany payables	(28,658)	(26,334)
Total liabilities	(227,253)	(225,354)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,213,331)	(1,208,533)
Total liabilities and stockholders' equity	(1,440,584)	(1,433,887)
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	3,612	3,826	4,235	3,716
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers, net of allowance for credit losses of $898 ($886 in 2018)	0	0
Income tax receivable	22,681	23,491
Securities purchased under agreements to resell		0
Securities owned, at fair value	1,443	1,358
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,548 and $6,841 respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	21,897	20,722
Operating Lease, Right-of-Use Asset	164,250
Subordinated loan receivable	112,558	112,558
Intangible assets	400	400
Goodwill	0	0
Other assets	1,975	2,581
Deferred tax assets	446	455
Investment in subsidiaries	539,316	546,704
Intercompany receivables	48,400	46,840
Total assets	916,978	758,935
Liabilities
Drafts payable	0	0
Bank call loans	0	0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	(8,444)	31,653
Operating Lease, Liability	205,229
Income tax payable	22,189	22,189
Senior secured notes, net of debt issuance costs of $840 ($904 in 2018)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	(411)	0
Intercompany payables	28,658	26,334
Total liabilities	247,221	80,176
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	669,757	678,759
Total liabilities and stockholders' equity	916,978	758,935
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	29,617	33,323	32,984	36,996
Deposits with clearing organizations	55,069	67,678
Receivable from brokers, dealers and clearing organizations	180,221	166,493
Receivable from customers, net of allowance for credit losses of $898 ($886 in 2018)	753,625	720,777
Income tax receivable	(702)	(702)
Securities purchased under agreements to resell	589	290
Securities owned, at fair value	910,025	836,226
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,548 and $6,841 respectively ($25,109 and $6,800, respectively, in 2018)	43,921	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	8,831	8,266
Operating Lease, Right-of-Use Asset	6,974
Subordinated loan receivable	0	0
Intangible assets	31,700	31,700
Goodwill	137,889	137,889
Other assets	125,471	110,052
Deferred tax assets	16,535	18,494
Investment in subsidiaries	0	0
Intercompany receivables	(8,252)	(6,299)
Total assets	2,291,513	2,168,245
Liabilities
Drafts payable	13,533	16,348
Bank call loans	0	15,000
Payable to brokers, dealers and clearing organizations	634,333	289,207
Payable to customers	334,376	336,616
Securities sold under agreements to repurchase	268,621	484,218
Securities sold but not yet purchased, at fair value	137,286	85,446
Accrued compensation	106,869	167,348
Accounts payable and other liabilities	53,744	55,823
Operating Lease, Liability	7,741
Income tax payable	45,332	42,878
Senior secured notes, net of debt issuance costs of $840 ($904 in 2018)	0	0
Subordinated indebtedness	112,558	112,558
Deferred tax liabilities, net	33,546	33,029
Intercompany payables	0	0
Total liabilities	1,747,939	1,638,471
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	543,574	529,774
Total liabilities and stockholders' equity	2,291,513	2,168,245
Parent
ASSETS
Cash and cash equivalents	49,844	53,526	5,496	$ 7,442
Deposits with clearing organizations	0	0
Receivable from brokers, dealers and clearing organizations	0	0
Receivable from customers, net of allowance for credit losses of $898 ($886 in 2018)	0	0
Income tax receivable	46,821	45,733
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,548 and $6,841 respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Operating Lease, Right-of-Use Asset	0
Subordinated loan receivable	0	0
Intangible assets	0	0
Goodwill	0	0
Other assets	211	135
Deferred tax assets	255	1
Investment in subsidiaries	676,344	661,837
Intercompany receivables	(13,818)	(14,211)
Total assets	759,657	747,021
Liabilities
Drafts payable	0	0
Bank call loans	0	0
Payable to brokers, dealers and clearing organizations	0	0
Payable to customers	0	0
Securities sold under agreements to repurchase	0	0
Securities sold but not yet purchased, at fair value	0	0
Accrued compensation	0	0
Accounts payable and other liabilities	3,570	163
Operating Lease, Liability	0
Income tax payable	2,440	2,440
Senior secured notes, net of debt issuance costs of $840 ($904 in 2018)	199,160	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	0
Total liabilities	205,170	201,699
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	554,487	545,322	$ 530,509
Total liabilities and stockholders' equity	$ 759,657	$ 747,021