Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
ASSETS
Cash and cash equivalents	$ 26,193	$ 90,675	$ 37,531	$ 48,154
Deposits with clearing organizations	73,730	67,678
Receivable from brokers, dealers and clearing organizations	157,734	166,493
Receivable from customers, net of allowance for credit losses of $941 ($886 in 2018)	754,927	720,777
Income tax receivable	2,333	1,014
Securities purchased under agreements to resell	0	290
Securities owned, at fair value	653,194	837,584
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $35,373 and $3,886 respectively ($25,109 and $6,800, respectively, in 2018)	43,920	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	32,085	28,988
Operating Lease, Right-of-Use Asset	164,899	0
Subordinated loan receivable	0	0
Intangible assets	32,100	32,100
Goodwill	137,889	137,889
Other assets	124,484	112,768
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,203,488	2,240,314
Liabilities
Drafts payable	17,794	16,348
Bank call loans	0	15,000
Payable to brokers, dealers and clearing organizations	364,595	289,207
Payable to customers	313,056	336,616
Securities sold under agreements to repurchase	253,720	484,218
Securities sold but not yet purchased, at fair value	121,880	85,446
Accrued compensation	149,862	167,348
Accounts payable and other liabilities	43,234	87,630
Operating Lease, Liability	208,748	0
Income tax payable	0	0
Senior secured notes, net of debt issuance costs of $533 ($904 in 2018)	149,467	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	14,505	14,083
Intercompany payables	0	0
Total liabilities	1,636,861	1,694,992
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	566,627	545,322
Total liabilities and stockholders' equity	2,203,488	2,240,314
Allowance for credit losses	941	886
Amounts pledged	359,151	517,951
Notes Receivable, Net Accumulated Amortization	35,373	25,109
Notes Receivable, Net Allowance for Uncollectibles	3,886	6,800
Net accumulated depreciation	92,690	89,182
Unamortized debt issuance expense	533	904
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 $941 ($886 in 2018)	0	0
Income tax receivable	(62,488)	(67,508)
Securities purchased under agreements to resell		0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $35,373 and $3,886 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	(10,931)	0
Deferred tax assets	(19,086)	(18,950)
Investment in subsidiaries	(1,478,582)	(1,208,541)
Intercompany receivables	(60,417)	(26,330)
Total assets	(1,744,062)	(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	(10,940)	(9)
Operating Lease, Liability	2
Income tax payable	(62,529)	(67,507)
Senior secured notes, net of debt issuance costs of $533 ($904 in 2018)	0	0
Subordinated indebtedness	(112,558)	(112,558)
Deferred tax liabilities, net	(19,041)	(18,946)
Intercompany payables	(27,727)	(26,334)
Total liabilities	(232,793)	(225,354)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,511,269)	(1,208,533)
Total liabilities and stockholders' equity	(1,744,062)	(1,433,887)
Parent | Reportable Legal Entities
ASSETS
Cash and cash equivalents	4,785	53,525
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 $941 ($886 in 2018)	0	0
Income tax receivable	49,333	45,733
Securities purchased under agreements to resell		0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $35,373 and $3,886 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	154	135
Deferred tax assets	103	1
Investment in subsidiaries	692,030	661,837
Intercompany receivables	0	(14,211)
Total assets	746,405	747,020
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	2,584	163
Operating Lease, Liability	0
Income tax payable	0	2,440
Senior secured notes, net of debt issuance costs of $533 ($904 in 2018)	149,467	199,096
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	27,727	0
Total liabilities	179,778	201,699
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	566,627	545,322
Total liabilities and stockholders' equity	746,405	747,021
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	534	3,826	4,653	3,716
Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	534	3,826
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 $941 ($886 in 2018)	0	0
Income tax receivable	15,488	23,491
Securities purchased under agreements to resell		0
Securities owned, at fair value	1,327	1,358
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $35,373 and $3,886 respectively ($25,109 and $6,800, respectively, in 2018)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	23,097	20,722
Operating Lease, Right-of-Use Asset	158,793
Subordinated loan receivable	112,558	112,558
Intangible assets	400	400
Goodwill	0	0
Other assets	12,612	2,581
Deferred tax assets	7,020	455
Investment in subsidiaries	761,924	546,704
Intercompany receivables	46,325	46,840
Total assets	1,140,078	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	90	31,653
Operating Lease, Liability	202,121
Income tax payable	40	22,189
Senior secured notes, net of debt issuance costs of $533 ($904 in 2018)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	26,334
Total liabilities	202,251	80,176
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	937,827	678,759
Total liabilities and stockholders' equity	1,140,078	758,935
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	20,874	33,324	$ 32,399	$ 36,996
Non-Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	20,874	33,324
Deposits with clearing organizations	73,730	67,678
Receivable from brokers, dealers and clearing organizations	157,734	166,493
Receivable from customers, net of allowance for credit losses of $941 ($886 in 2018)	754,927	720,777
Income tax receivable	0	(702)
Securities purchased under agreements to resell		290
Securities owned, at fair value	651,867	836,226
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $35,373 and $3,886 respectively ($25,109 and $6,800, respectively, in 2018)	43,920	44,058
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	8,988	8,266
Operating Lease, Right-of-Use Asset	6,106
Subordinated loan receivable	0	0
Intangible assets	31,700	31,700
Goodwill	137,889	137,889
Other assets	122,649	110,052
Deferred tax assets	11,963	18,494
Investment in subsidiaries	24,628	0
Intercompany receivables	14,092	(6,299)
Total assets	2,061,067	2,168,246
Liabilities
Drafts payable	17,794	16,348
Bank call loans	0	15,000
Payable to brokers, dealers and clearing organizations	364,595	289,207
Payable to customers	313,056	336,616
Securities sold under agreements to repurchase	253,720	484,218
Securities sold but not yet purchased, at fair value	121,880	85,446
Accrued compensation	149,862	167,348
Accounts payable and other liabilities	51,500	55,823
Operating Lease, Liability	6,625
Income tax payable	62,489	42,878
Senior secured notes, net of debt issuance costs of $533 ($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	1,487,625	1,638,471
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	573,442	529,774
Total liabilities and stockholders' equity	$ 2,061,067	$ 2,168,245