Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
ASSETS
Cash and cash equivalents	$ 51,237			$ 79,550	$ 71,307	$ 90,675
Deposits with clearing organizations	90,536			48,415
Receivable from brokers, dealers and clearing organizations	315,947			163,293
Receivable from customers, net of allowance for credit losses of $493 ($451 in 2019)	860,140			796,934
Income tax receivable	1,789			5,170
Securities owned, at fair value	502,029			799,719
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $34,022 and $3,903 respectively ($38,355 and $3,673, respectively, in 2019)	44,114			43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	30,418			31,377
Operating Lease, Right-of-Use Asset	160,358			160,297
Subordinated loan receivable	0			0
Intangible assets	32,100			32,100
Goodwill	137,889			137,889
Other assets	137,760			166,341
Deferred tax assets	0			0
Investment in subsidiaries	0			0
Intercompany receivables	0			0
Total assets	2,364,317			2,464,755
Liabilities
Drafts payable	37,374			0
Bank call loans	13,000			0
Payable to brokers, dealers and clearing organizations	216,111			520,975
Payable to customers	416,914			334,735
Securities sold under agreements to repurchase	154,155			287,265
Securities sold but not yet purchased, at fair value	356,589			100,571
Accrued compensation	146,988			207,358
Accounts payable and other liabilities	43,219			44,725
Operating Lease, Liability	202,028			203,140
Income tax payable	0			0
Senior secured notes, net of debt issuance costs of $384 ($485 in 2019)	148,190			149,515
Subordinated indebtedness	0			0
Deferred tax liabilities, net	28,612			23,749
Intercompany payables	0			0
Total liabilities	1,763,180			1,872,033
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	601,137			592,722	563,610
Total liabilities and stockholders' equity	2,364,317			2,464,755
Allowance for credit losses	493			451
Amounts pledged	357,323			357,120
Notes Receivable, Net Accumulated Amortization	34,022			38,355
Notes Receivable, Net Allowance for Uncollectibles	3,903	$ 3,908	$ 3,673	3,673
Net accumulated depreciation	98,885			94,773
Unamortized debt issuance expense	384			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 $493 ($451 in 2019)	0			0
Income tax receivable	(57,005)			(53,855)
Securities owned, at fair value	0			0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $34,022 and $3,903 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	2,440			(9,501)
Investment in subsidiaries	(1,378,455)			(1,485,739)
Intercompany receivables	(26,336)			(70,798)
Total assets	(1,572,126)			(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	20			(10,565)
Operating Lease, Liability	0			0
Income tax payable	(57,005)			(743)
Senior secured notes, net of debt issuance costs of $384 ($485 in 2019)	0			0
Subordinated indebtedness	(112,558)			(112,558)
Deferred tax liabilities, net	2,443			(9,797)
Intercompany payables	(26,335)			(299)
Total liabilities	(193,435)			(133,962)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,378,691)			(1,604,068)
Total liabilities and stockholders' equity	(1,572,126)			(1,738,030)
Parent | Reportable Legal Entities
ASSETS
Cash and cash equivalents	267			4,811
Deposits with clearing organizations	0			0
Receivable from brokers, dealers and clearing organizations				0
Receivable from customers, net of allowance for credit losses of $493 ($451 in 2019)	0			0
Income tax receivable	44,125			42,556
Securities owned, at fair value	0			0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $34,022 and $3,903 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	179			154
Deferred tax assets	0			4
Investment in subsidiaries	757,878			697,093
Intercompany receivables	(50,836)			2,875
Total assets	751,822			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	55			5,166
Operating Lease, Liability	0			0
Income tax payable	2,440			0
Senior secured notes, net of debt issuance costs of $384 ($485 in 2019)	148,190			149,515
Subordinated indebtedness	0			0
Deferred tax liabilities, net	0			0
Intercompany payables	0			299
Total liabilities	150,685			154,980
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	601,137			592,722
Total liabilities and stockholders' equity	751,822			747,702
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	185			2,179	475	3,826
Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	185			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 $493 ($451 in 2019)	0			0
Income tax receivable	15,371			16,469
Securities owned, at fair value	1,422			1,352
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $34,022 and $3,903 respectively ($38,355 and $3,673, respectively, in 2019)	0			0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	21,761			22,537
Operating Lease, Right-of-Use Asset	155,192			153,780
Subordinated loan receivable	112,558			112,558
Intangible assets	400			400
Goodwill	0			0
Other assets	1,368			11,798
Deferred tax assets	(26)			7,048
Investment in subsidiaries	620,581			763,990
Intercompany receivables	72,619			67,923
Total assets	1,001,431			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	(10,305)			75
Operating Lease, Liability	196,604			196,234
Income tax payable	22,189			40
Senior secured notes, net of debt issuance costs of $384 ($485 in 2019)	0			0
Subordinated indebtedness	0			0
Deferred tax liabilities, net	(7,377)			0
Intercompany payables	26,335			0
Total liabilities	227,446			196,349
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	773,985			963,685
Total liabilities and stockholders' equity	1,001,431			1,160,034
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	50,785			72,560	$ 32,597	$ 33,323
Non-Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	50,785			72,560
Deposits with clearing organizations	90,536			48,415
Receivable from brokers, dealers and clearing organizations	315,947			158,231
Receivable from customers, net of allowance for credit losses of $493 ($451 in 2019)	860,140			796,934
Income tax receivable	(702)			0
Securities owned, at fair value	500,607			798,367
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $34,022 and $3,903 respectively ($38,355 and $3,673, respectively, in 2019)	44,114			43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	8,657			8,840
Operating Lease, Right-of-Use Asset	5,166			6,517
Subordinated loan receivable	0			0
Intangible assets	31,700			31,700
Goodwill	137,889			137,889
Other assets	136,216			164,821
Deferred tax assets	(2,414)			2,449
Investment in subsidiaries	(4)			24,656
Intercompany receivables	4,553			0
Total assets	2,183,190			2,295,049
Liabilities
Drafts payable	37,374
Bank call loans	13,000
Payable to brokers, dealers and clearing organizations	216,111			520,975
Payable to customers	416,914			334,735
Securities sold under agreements to repurchase	154,155			287,265
Securities sold but not yet purchased, at fair value	356,589			100,571
Accrued compensation	146,988			207,358
Accounts payable and other liabilities	53,449			50,049
Operating Lease, Liability	5,424			6,906
Income tax payable	32,376			703
Senior secured notes, net of debt issuance costs of $384 ($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	1,578,484			1,654,666
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	604,706			640,383
Total liabilities and stockholders' equity	$ 2,183,190			$ 2,295,049