Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
ASSETS
Cash and cash equivalents	$ 38,961,000	$ 48,154,000	$ 116,582,000	$ 64,913,000
Deposits with clearing organizations	56,864,000	42,222,000
Receivable from brokers, dealers and clearing organizations	204,527,000	187,115,000
Receivable from customers, net of allowance for credit losses of $848 ($769 in 2017)	835,708,000	848,226,000
Income tax receivable	2,818,000	2,939,000
Securities purchased under agreements to resell, at fair value	6,738,000	658,000
Securities owned, at fair value	1,020,404,000	926,597,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,788 and $8,051, respectively ($24,705 and $7,975, respectively, in 2017)	42,410,000	40,520,000
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	27,994,000	27,187,000
Subordinated loan receivable	0	0
Intangible assets	32,100,000	31,700,000
Goodwill	137,889,000	137,889,000
Other assets	114,867,000	145,310,000
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,521,280,000	2,438,517,000
Liabilities
Drafts payable	21,632,000	42,412,000
Bank call loans	107,500,000	118,300,000
Payable to brokers, dealers and clearing organizations	282,542,000	211,483,000
Payable to customers	373,664,000	385,907,000
Securities sold under agreements to repurchase	599,151,000	586,478,000
Securities sold but not yet purchased, at fair value	162,042,000	94,486,000
Accrued compensation	128,985,000	173,116,000
Accounts payable and other liabilities	94,353,000	92,495,000
Income tax payable	0	0
Senior secured notes, net of debt issuance costs of $1,034 ($1,163 in 2017)	198,966,000	198,837,000
Subordinated indebtedness	0	0
Deferred tax liabilities, net	13,804,000	11,092,000
Intercompany payables	0	0
Total liabilities	1,982,639,000	1,914,606,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	538,292,000	523,550,000
Non-controlling interest	349,000	361,000
Total stockholders' equity	538,641,000	523,911,000	501,114,000
Total liabilities and stockholders' equity	2,521,280,000	2,438,517,000
Allowance for credit losses	848,000	769,000
Amounts pledged	692,286,000	655,683,000
Notes Receivable, Net Accumulated Amortization	24,788,000	24,705,000
Notes Receivable, Net Allowance for Uncollectibles	8,051,000	7,975,000
Net accumulated depreciation	85,815,000	82,826,000
Unamortized debt issuance expense	1,034,000	1,163,000
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 $848 ($769 in 2017)	0	0
Income tax receivable	(68,951,000)	(69,084,000)
Securities purchased under agreements to resell, at fair value	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,788 and $8,051, respectively ($24,705 and $7,975, respectively, in 2017)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Subordinated loan receivable	(112,558,000)	(112,558,000)
Intangible assets	0	0
Goodwill	0	0
Other assets	0	0
Deferred tax assets	(18,881,000)	(21,965,000)
Investment in subsidiaries	(1,169,979,000)	(1,130,571,000)
Intercompany receivables	(98,470,000)	(135,586,000)
Total assets	(1,468,839,000)	(1,469,764,000)
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	0	0
Income tax payable	(68,951,000)	(69,084,000)
Senior secured notes, net of debt issuance costs of $1,034 ($1,163 in 2017)	0	0
Subordinated indebtedness	(112,558,000)	(112,558,000)
Deferred tax liabilities, net	(18,881,000)	(21,965,000)
Intercompany payables	(98,470,000)	(135,586,000)
Total liabilities	(298,860,000)	(339,193,000)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,169,979,000)	(1,130,571,000)
Non-controlling interest	0	0
Total stockholders' equity	(1,169,979,000)	(1,130,571,000)
Total liabilities and stockholders' equity	(1,468,839,000)	(1,469,764,000)
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	4,588,000	3,716,000	3,232,000	10,284,000
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 $848 ($769 in 2017)	0	0
Income tax receivable	24,904,000	26,025,000
Securities purchased under agreements to resell, at fair value	0	0
Securities owned, at fair value	1,369,000	1,386,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,788 and $8,051, respectively ($24,705 and $7,975, respectively, in 2017)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	20,883,000	20,221,000
Subordinated loan receivable	112,558,000	112,558,000
Intangible assets	400,000	0
Goodwill	0	0
Other assets	2,580,000	2,573,000
Deferred tax assets	118,000	0
Investment in subsidiaries	526,774,000	507,747,000
Intercompany receivables	49,627,000	83,437,000
Total assets	743,801,000	757,663,000
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	33,496,000	33,994,000
Income tax payable	22,189,000	22,189,000
Senior secured notes, net of debt issuance costs of $1,034 ($1,163 in 2017)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	17,000
Intercompany payables	28,141,000	62,163,000
Total liabilities	83,826,000	118,363,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	659,975,000	639,300,000
Non-controlling interest	0	0
Total stockholders' equity	659,975,000	639,300,000
Total liabilities and stockholders' equity	743,801,000	757,663,000
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	33,705,000	36,996,000	39,697,000	54,400,000
Deposits with clearing organizations	56,864,000	42,222,000
Receivable from brokers, dealers and clearing organizations	204,527,000	187,115,000
Receivable from customers, net of allowance for credit losses of $848 ($769 in 2017)	835,708,000	848,226,000
Income tax receivable	0	0
Securities purchased under agreements to resell, at fair value	6,738,000	658,000
Securities owned, at fair value	1,019,035,000	925,211,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,788 and $8,051, respectively ($24,705 and $7,975, respectively, in 2017)	42,410,000	40,520,000
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	7,111,000	6,966,000
Subordinated loan receivable	0	0
Intangible assets	31,700,000	31,700,000
Goodwill	137,889,000	137,889,000
Other assets	112,167,000	142,604,000
Deferred tax assets	18,641,000	18,463,000
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,506,495,000	2,418,570,000
Liabilities
Drafts payable	21,632,000	42,412,000
Bank call loans	107,500,000	118,300,000
Payable to brokers, dealers and clearing organizations	282,542,000	211,483,000
Payable to customers	373,664,000	385,907,000
Securities sold under agreements to repurchase	599,151,000	586,478,000
Securities sold but not yet purchased, at fair value	162,042,000	94,486,000
Accrued compensation	128,985,000	173,116,000
Accounts payable and other liabilities	60,732,000	51,280,000
Income tax payable	44,322,000	44,455,000
Senior secured notes, net of debt issuance costs of $1,034 ($1,163 in 2017)	0	0
Subordinated indebtedness	112,558,000	112,558,000
Deferred tax liabilities, net	32,685,000	33,040,000
Intercompany payables	70,329,000	73,423,000
Total liabilities	1,996,142,000	1,926,938,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	510,004,000	491,271,000
Non-controlling interest	349,000	361,000
Total stockholders' equity	510,353,000	491,632,000
Total liabilities and stockholders' equity	2,506,495,000	2,418,570,000
Parent
ASSETS
Cash and cash equivalents	668,000	7,442,000	73,653,000	$ 229,000
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 $848 ($769 in 2017)	0	0
Income tax receivable	46,865,000	45,998,000
Securities purchased under agreements to resell, at fair value	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,788 and $8,051, respectively ($24,705 and $7,975, respectively, in 2017)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	0	0
Subordinated loan receivable	0	0
Intangible assets	0	0
Goodwill	0	0
Other assets	120,000	133,000
Deferred tax assets	122,000	3,502,000
Investment in subsidiaries	643,205,000	622,824,000
Intercompany receivables	48,843,000	52,149,000
Total assets	739,823,000	732,048,000
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	125,000	7,221,000
Income tax payable	2,440,000	2,440,000
Senior secured notes, net of debt issuance costs of $1,034 ($1,163 in 2017)	198,966,000	198,837,000
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	0
Total liabilities	201,531,000	208,498,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	538,292,000	523,550,000
Non-controlling interest	0	0
Total stockholders' equity	538,292,000	523,550,000	$ 499,200,000
Total liabilities and stockholders' equity	$ 739,823,000	$ 732,048,000