Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
ASSETS
Cash and cash equivalents	$ 32,080			$ 79,550	$ 26,193	$ 90,675
Deposits with clearing organizations	77,202			48,415
Receivable from brokers, dealers and clearing organizations	207,256			163,293
Receivable from customers, net of allowance for credit losses of $418 ($451 in 2019)	1,095,035			796,934
Income tax receivable	2,992			5,170
Securities owned, at fair value	646,680			799,719
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $37,224 and $4,190 respectively ($38,355 and $3,673, respectively, in 2019)	44,723			43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	28,916			31,377
Operating Lease, Right-of-Use Asset	154,666			160,297
Subordinated loan receivable	0			0
Intangible assets	32,100			32,100
Goodwill	137,889			137,889
Other assets	149,079			166,341
Deferred tax assets	0			0
Investment in subsidiaries	0			0
Intercompany receivables	0			0
Total assets	2,608,618			2,464,755
Liabilities
Drafts payable	18,251			0
Bank call loans	156,900			0
Payable to brokers, dealers and clearing organizations	366,949			520,975
Payable to customers	380,746			334,735
Securities sold under agreements to repurchase	252,827			287,265
Securities sold but not yet purchased, at fair value	222,449			100,571
Accrued compensation	201,616			207,358
Accounts payable and other liabilities	43,473			44,725
Operating Lease, Liability	195,348			203,140
Income tax payable	0			0
Senior secured notes, net of debt issuance costs of $1,136 ($485 in 2019)	123,864			149,515
Subordinated indebtedness	0			0
Deferred tax liabilities, net	30,979			23,749
Intercompany payables	0			0
Total liabilities	1,993,402			1,872,033
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	615,216			592,722	566,627
Total liabilities and stockholders' equity	2,608,618			2,464,755
Allowance for credit losses	418			451
Amounts pledged	486,165			357,120
Notes Receivable, Net Accumulated Amortization	37,224			38,355
Notes Receivable, Net Allowance for Uncollectibles	4,190	$ 3,903	$ 3,673	3,673
Net accumulated depreciation	100,968			94,773
Unamortized debt issuance expense	1,136			485
Eliminations
ASSETS
Cash and cash equivalents	0			0	0	0
Deposits with clearing organizations	0			0
Receivable from brokers, dealers and clearing organizations	0			5,062
Receivable from customers, net of allowance for credit losses of $418 ($451 in 2019)	0			0
Income tax receivable	(57,365)			(53,855)
Securities owned, at fair value	1			0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $37,224 and $4,190 respectively ($38,355 and $3,673, respectively, in 2019)	0			0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0			0
Operating Lease, Right-of-Use Asset	0			0
Subordinated loan receivable	(112,767)			(112,767)
Intangible assets	0			0
Goodwill	0			0
Other assets	(3)			(10,432)
Deferred tax assets	2,696			(9,501)
Investment in subsidiaries	(1,372,882)			(1,485,739)
Intercompany receivables	(26,331)			(70,798)
Total assets	(1,566,651)			(1,738,030)
Liabilities
Drafts payable	0
Bank call loans	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	21			(10,565)
Operating Lease, Liability	0			0
Income tax payable	(59,398)			(743)
Senior secured notes, net of debt issuance costs of $1,136 ($485 in 2019)	0			0
Subordinated indebtedness	(112,558)			(112,558)
Deferred tax liabilities, net	4,731			(9,797)
Intercompany payables	(26,335)			(299)
Total liabilities	(193,539)			(133,962)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,373,112)			(1,604,068)
Total liabilities and stockholders' equity	(1,566,651)			(1,738,030)
Parent | Reportable Legal Entities
ASSETS
Cash and cash equivalents	3,864			4,811
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 $418 ($451 in 2019)	0			0
Income tax receivable	46,136			42,556
Securities owned, at fair value	0			0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $37,224 and $4,190 respectively ($38,355 and $3,673, respectively, in 2019)	0			0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0			0
Operating Lease, Right-of-Use Asset	0			0
Subordinated loan receivable	209			209
Intangible assets	0			0
Goodwill	0			0
Other assets	(786)			154
Deferred tax assets	0			4
Investment in subsidiaries	779,573			697,093
Intercompany receivables	(87,257)			2,875
Total assets	741,739			747,702
Liabilities
Drafts payable	0
Bank call loans	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	219			5,166
Operating Lease, Liability	0			0
Income tax payable	2,440			0
Senior secured notes, net of debt issuance costs of $1,136 ($485 in 2019)	123,864			149,515
Subordinated indebtedness	0			0
Deferred tax liabilities, net	0			0
Intercompany payables	0			299
Total liabilities	126,523			154,980
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	615,216			592,722
Total liabilities and stockholders' equity	741,739			747,702
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	188			2,179	534	3,826
Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	188			2,179
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 $418 ($451 in 2019)	0			0
Income tax receivable	14,923			16,469
Securities owned, at fair value	1,438			1,352
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $37,224 and $4,190 respectively ($38,355 and $3,673, respectively, in 2019)	0			0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	20,989			22,537
Operating Lease, Right-of-Use Asset	150,115			153,780
Subordinated loan receivable	112,558			112,558
Intangible assets	400			400
Goodwill	0			0
Other assets	1,367			11,798
Deferred tax assets	(28)			7,048
Investment in subsidiaries	593,310			763,990
Intercompany receivables	121,365			67,923
Total assets	1,016,625			1,160,034
Liabilities
Drafts payable	0
Bank call loans	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	(10,320)			75
Operating Lease, Liability	190,603			196,234
Income tax payable	22,189			40
Senior secured notes, net of debt issuance costs of $1,136 ($485 in 2019)	0			0
Subordinated indebtedness	0			0
Deferred tax liabilities, net	(7,298)			0
Intercompany payables	26,335			0
Total liabilities	221,509			196,349
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	795,116			963,685
Total liabilities and stockholders' equity	1,016,625			1,160,034
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	28,028			72,560	$ 20,874	$ 33,324
Non-Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	28,028			72,560
Deposits with clearing organizations	77,202			48,415
Receivable from brokers, dealers and clearing organizations	207,256			158,231
Receivable from customers, net of allowance for credit losses of $418 ($451 in 2019)	1,095,035			796,934
Income tax receivable	(702)			0
Securities owned, at fair value	645,241			798,367
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $37,224 and $4,190 respectively ($38,355 and $3,673, respectively, in 2019)	44,723			43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	7,927			8,840
Operating Lease, Right-of-Use Asset	4,551			6,517
Subordinated loan receivable	0			0
Intangible assets	31,700			31,700
Goodwill	137,889			137,889
Other assets	148,501			164,821
Deferred tax assets	(2,668)			2,449
Investment in subsidiaries	(1)			24,656
Intercompany receivables	(7,777)			0
Total assets	2,416,905			2,295,049
Liabilities
Drafts payable	18,251
Bank call loans	156,900
Payable to brokers, dealers and clearing organizations	366,949			520,975
Payable to customers	380,746			334,735
Securities sold under agreements to repurchase	252,827			287,265
Securities sold but not yet purchased, at fair value	222,449			100,571
Accrued compensation	201,616			207,358
Accounts payable and other liabilities	53,553			50,049
Operating Lease, Liability	4,745			6,906
Income tax payable	34,769			703
Senior secured notes, net of debt issuance costs of $1,136 ($485 in 2019)	0			0
Subordinated indebtedness	112,558			112,558
Deferred tax liabilities, net	33,546			33,546
Intercompany payables	0			0
Total liabilities	1,838,909			1,654,666
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	577,996			640,383
Total liabilities and stockholders' equity	$ 2,416,905			$ 2,295,049