Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 71,307	$ 90,675	$ 38,961	$ 48,154
Deposits with clearing organizations	65,491	67,678
Receivable from brokers, dealers and clearing organizations	167,931	166,493
Receivable from customers, net of allowance for credit losses of $920 ($886 in 2018)	807,581	720,777
Income tax receivable	0	1,014
Securities purchased under agreements to resell	39	290
Securities owned, at fair value	995,370	837,584
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $32,369 and $6,827 respectively ($25,109 and $6,800, respectively, in 2018)	43,413	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	32,327	28,988
Operating Lease, Right-of-Use Asset	166,612	0
Subordinated loan receivable	0	0
Intangible assets	32,100	32,100
Goodwill	137,889	137,889
Other assets	132,356	112,768
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,652,416	2,240,314
Liabilities
Drafts payable	17,456	16,348
Bank call loans	50,100	15,000
Payable to brokers, dealers and clearing organizations	418,719	289,207
Payable to customers	325,754	336,616
Securities sold under agreements to repurchase	521,837	484,218
Securities sold but not yet purchased, at fair value	155,979	85,446
Accrued compensation	130,879	167,348
Accounts payable and other liabilities	42,709	87,630
Operating Lease, Liability	210,255	0
Income tax payable	849	0
Senior secured notes, net of debt issuance costs of $775 ($904 in 2018)	199,225	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	15,044	14,083
Intercompany payables	0	0
Total liabilities	2,088,806	1,694,992
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	563,610	545,322
Total liabilities and stockholders' equity	2,652,416	2,240,314
Allowance for credit losses	920	886
Amounts pledged	547,326	517,951
Notes Receivable, Net Accumulated Amortization	32,369	25,109
Notes Receivable, Net Allowance for Uncollectibles	6,827	6,800
Net accumulated depreciation	90,633	89,182
Unamortized debt issuance expense	775	904
Operating Lease Right-of-use Asset Accumulated Amortization	13,000
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 $920 ($886 in 2018)	0	0
Income tax receivable	(69,462)	(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 $32,369 and $6,827 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	(17,699)	(18,950)
Investment in subsidiaries	(1,272,950)	(1,208,541)
Intercompany receivables	(26,648)	(26,330)
Total assets	(1,499,317)	(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	(9)	(9)
Operating Lease, Liability	0
Income tax payable	(69,463)	(67,507)
Senior secured notes, net of debt issuance costs of $775 ($904 in 2018)	0	0
Subordinated indebtedness	(112,558)	(112,558)
Deferred tax liabilities, net	(17,694)	(18,946)
Intercompany payables	193,967	(26,334)
Total liabilities	(5,757)	(225,354)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,493,560)	(1,208,533)
Total liabilities and stockholders' equity	(1,499,317)	(1,433,887)
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	475	3,826	4,588	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 $920 ($886 in 2018)	0	0
Income tax receivable	22,247	23,491
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	1,493	1,358
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $32,369 and $6,827 respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	23,297	20,722
Operating Lease, Right-of-Use Asset	160,202
Subordinated loan receivable	112,558	112,558
Intangible assets	400	400
Goodwill	0	0
Other assets	1,956	2,581
Deferred tax assets	(29)	455
Investment in subsidiaries	580,920	546,704
Intercompany receivables	46,960	46,840
Total assets	950,479	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	(10,302)	31,653
Operating Lease, Liability	203,240
Income tax payable	22,189	22,189
Senior secured notes, net of debt issuance costs of $775 ($904 in 2018)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	(808)	0
Intercompany payables	(193,967)	26,334
Total liabilities	20,352	80,176
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	930,127	678,759
Total liabilities and stockholders' equity	950,479	758,935
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	32,597	33,323	33,705	36,996
Deposits with clearing organizations	65,491	67,678
Receivable from brokers, dealers and clearing organizations	167,931	166,493
Receivable from customers, net of allowance for credit losses of $920 ($886 in 2018)	807,581	720,777
Income tax receivable	(702)	(702)
Securities purchased under agreements to resell	39	290
Securities owned, at fair value	993,877	836,226
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $32,369 and $6,827 respectively ($25,109 and $6,800, respectively, in 2018)	43,413	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	9,030	8,266
Operating Lease, Right-of-Use Asset	6,410
Subordinated loan receivable	0	0
Intangible assets	31,700	31,700
Goodwill	137,889	137,889
Other assets	130,275	110,052
Deferred tax assets	17,573	18,494
Investment in subsidiaries	0	0
Intercompany receivables	(7,420)	(6,299)
Total assets	2,435,684	2,168,245
Liabilities
Drafts payable	17,456	16,348
Bank call loans	50,100	15,000
Payable to brokers, dealers and clearing organizations	418,719	289,207
Payable to customers	325,754	336,616
Securities sold under agreements to repurchase	521,837	484,218
Securities sold but not yet purchased, at fair value	155,979	85,446
Accrued compensation	130,879	167,348
Accounts payable and other liabilities	52,725	55,823
Operating Lease, Liability	7,015
Income tax payable	45,683	42,878
Senior secured notes, net of debt issuance costs of $775 ($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	1,872,251	1,638,471
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	563,433	529,774
Total liabilities and stockholders' equity	2,435,684	2,168,245
Parent
ASSETS
Cash and cash equivalents	38,235	53,526	$ 668	$ 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 $920 ($886 in 2018)	0	0
Income tax receivable	47,917	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 $32,369 and $6,827 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	125	135
Deferred tax assets	155	1
Investment in subsidiaries	692,030	661,837
Intercompany receivables	(12,892)	(14,211)
Total assets	765,570	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	295	163
Operating Lease, Liability	0
Income tax payable	2,440	2,440
Senior secured notes, net of debt issuance costs of $775 ($904 in 2018)	199,225	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	0
Total liabilities	201,960	201,699
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	563,610	545,322
Total liabilities and stockholders' equity	$ 765,570	$ 747,021