Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
ASSETS
Cash and cash equivalents	$ 23,540		$ 79,550	$ 83,073	$ 90,675
Deposits with clearing organizations	119,580		48,415
Receivable from brokers, dealers and clearing organizations	198,417		163,293
Receivable from customers, net of allowance for credit losses of $528 ($451 in 2019)	964,772		796,934
Income tax receivable	6,283		5,170
Securities owned, at fair value	309,346		799,719
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $31,188 and $3,908 respectively ($38,355 and $3,673, respectively, in 2019)	44,960		43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	30,646		31,377
Operating Lease, Right-of-Use Asset	156,040		160,297
Subordinated loan receivable	0		0
Intangible assets	32,100		32,100
Goodwill	137,889		137,889
Other assets	116,060		166,341
Deferred tax assets	0		0
Investment in subsidiaries	0		0
Intercompany receivables	0		0
Total assets	2,139,633		2,464,755
Liabilities
Drafts payable	19,066		0
Bank call loans	203,100		0
Payable to brokers, dealers and clearing organizations	239,583		520,975
Payable to customers	413,646		334,735
Securities sold under agreements to repurchase	124,103		287,265
Securities sold but not yet purchased, at fair value	34,639		100,571
Accrued compensation	102,407		207,358
Accounts payable and other liabilities	43,645		44,725
Operating Lease, Liability	197,728		203,140
Income tax payable	0		0
Senior secured notes, net of debt issuance costs of $432 ($485 in 2019)	148,142		149,515
Subordinated indebtedness	0		0
Deferred tax liabilities, net	26,844		23,749
Intercompany payables	0		0
Total liabilities	1,552,903		1,872,033
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	586,730		592,722	554,487
Total liabilities and stockholders' equity	2,139,633		2,464,755
Allowance for credit losses	528		451
Amounts pledged	150,547		357,120
Notes Receivable, Net Accumulated Amortization	31,188		38,355
Notes Receivable, Net Allowance for Uncollectibles	3,908	$ 3,673	3,673
Net accumulated depreciation	96,742		94,773
Unamortized debt issuance expense	432		485
Eliminations
ASSETS
Cash and cash equivalents	0		0	0	0
Deposits with clearing organizations	0		0
Receivable from brokers, dealers and clearing organizations			5,062
Receivable from customers, net of allowance for credit losses of $528 ($451 in 2019)	0		0
Income tax receivable	(52,145)		(53,855)
Securities owned, at fair value	0		0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $31,188 and $3,908 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	85		0
Subordinated loan receivable	(112,767)		(112,767)
Intangible assets	0		0
Goodwill	0		0
Other assets	1		(10,432)
Deferred tax assets	504		(9,501)
Investment in subsidiaries	(1,336,415)		(1,485,739)
Intercompany receivables	(26,333)		(70,798)
Total assets	(1,527,070)		(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	22		(10,565)
Operating Lease, Liability	0		0
Income tax payable	(52,145)		(743)
Senior secured notes, net of debt issuance costs of $432 ($485 in 2019)	0		0
Subordinated indebtedness	(112,558)		(112,558)
Deferred tax liabilities, net	507		(9,797)
Intercompany payables	(26,335)		(299)
Total liabilities	(190,509)		(133,962)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,336,561)		(1,604,068)
Total liabilities and stockholders' equity	(1,527,070)		(1,738,030)
Parent | Reportable Legal Entities
ASSETS
Cash and cash equivalents	1,950		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 $528 ($451 in 2019)	0		0
Income tax receivable	43,061		42,556
Securities owned, at fair value	0		0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $31,188 and $3,908 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	144		154
Deferred tax assets	238		4
Investment in subsidiaries	737,510		697,093
Intercompany receivables	(43,139)		2,875
Total assets	739,973		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	2,661		5,166
Operating Lease, Liability	0		0
Income tax payable	2,440		0
Senior secured notes, net of debt issuance costs of $432 ($485 in 2019)	148,142		149,515
Subordinated indebtedness	0		0
Deferred tax liabilities, net	0		0
Intercompany payables	0		299
Total liabilities	153,243		154,980
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	586,730		592,722
Total liabilities and stockholders' equity	739,973		747,702
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	67		2,179	3,612	3,826
Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	67		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 $528 ($451 in 2019)	0		0
Income tax receivable	16,068		16,469
Securities owned, at fair value	1,339		1,352
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $31,188 and $3,908 respectively ($38,355 and $3,673, respectively, in 2019)	0		0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	21,741		22,537
Operating Lease, Right-of-Use Asset	149,929		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	(9)		7,048
Investment in subsidiaries	598,905		763,990
Intercompany receivables	73,322		67,923
Total assets	975,688		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	(10,294)		75
Operating Lease, Liability	191,294		196,234
Income tax payable	22,189		40
Senior secured notes, net of debt issuance costs of $432 ($485 in 2019)	0		0
Subordinated indebtedness	0		0
Deferred tax liabilities, net	(7,209)		0
Intercompany payables	26,335		0
Total liabilities	222,315		196,349
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	753,373		963,685
Total liabilities and stockholders' equity	975,688		1,160,034
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	21,523		72,560	$ 29,617	$ 33,323
Non-Guarantor Subsidiaries | Reportable Legal Entities
ASSETS
Cash and cash equivalents	21,523		72,560
Deposits with clearing organizations	119,580		48,415
Receivable from brokers, dealers and clearing organizations	198,417		158,231
Receivable from customers, net of allowance for credit losses of $528 ($451 in 2019)	964,772		796,934
Income tax receivable	(701)		0
Securities owned, at fair value	308,007		798,367
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $31,188 and $3,908 respectively ($38,355 and $3,673, respectively, in 2019)	44,960		43,670
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	8,905		8,840
Operating Lease, Right-of-Use Asset	6,026		6,517
Subordinated loan receivable	0		0
Intangible assets	31,700		31,700
Goodwill	137,889		137,889
Other assets	114,547		164,821
Deferred tax assets	(733)		2,449
Investment in subsidiaries	0		24,656
Intercompany receivables	(3,850)		0
Total assets	1,951,042		2,295,049
Liabilities
Drafts payable	19,066
Bank call loans	203,100
Payable to brokers, dealers and clearing organizations	239,583		520,975
Payable to customers	413,646		334,735
Securities sold under agreements to repurchase	124,103		287,265
Securities sold but not yet purchased, at fair value	34,639		100,571
Accrued compensation	102,407		207,358
Accounts payable and other liabilities	51,256		50,049
Operating Lease, Liability	6,434		6,906
Income tax payable	27,516		703
Senior secured notes, net of debt issuance costs of $432 ($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	1,367,854		1,654,666
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	583,188		640,383
Total liabilities and stockholders' equity	$ 1,951,042		$ 2,295,049