Supplemental Guarantor Condensed Consolidated Financial Statements - Condensed Consolidating Balance Sheet (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash and cash equivalents	$ 48,154,000	$ 64,913,000	$ 63,364,000	$ 63,807,000
Deposits with clearing organizations	42,222,000	38,185,000
Receivable from brokers, dealers and clearing organizations	187,115,000	214,934,000
Receivable from customers, net of allowance for credit losses of $769 ($794 in 2016)	848,226,000	847,386,000
Income tax receivable	2,939,000	5,816,000
Securities purchased under agreements to resell	658,000	24,006,000
Securities owned, at fair value	926,597,000	707,108,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,705 and $7,975, respectively ($24,826 and $6,784, respectively, in 2016)	40,520,000	30,099,000
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	27,187,000	27,233,000
Subordinated loan receivable	0	0
Intangible assets	31,700,000	31,700,000
Goodwill	137,889,000	137,889,000
Other assets	145,310,000	107,661,000
Deferred tax assets	0	0
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,438,517,000	2,236,930,000
Liabilities
Drafts payable	42,412,000	39,228,000
Bank call loans	118,300,000	145,800,000
Payable to brokers, dealers and clearing organizations	211,483,000	221,389,000
Payable to customers	385,907,000	449,946,000
Securities sold under agreements to repurchase	586,478,000	378,084,000
Securities sold but not yet purchased, at fair value	94,486,000	85,050,000
Accrued compensation	173,116,000	145,053,000
Accounts payable and other liabilities	92,495,000	96,557,000
Income tax payable	0	0
Senior secured notes, net of debt issuance costs of $1,163 ($648 in 2016)	198,837,000	149,352,000
Subordinated indebtedness	0	0
Deferred tax liabilities, net	11,092,000	13,137,000
Intercompany payables	0	0
Total liabilities	1,914,606,000	1,723,596,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	523,550,000	510,703,000
Noncontrolling interest	361,000	2,631,000
Total stockholders' equity	523,911,000	513,334,000	525,082,000
Total liabilities and stockholders' equity	2,438,517,000	2,236,930,000
Allowance for credit losses	769,000	794,000
Amounts pledged	655,683,000	438,385,000
Notes Receivable, Net Accumulated Amortization	24,705,000	24,826,000
Notes Receivable, Net Allowance for Uncollectibles	7,975,000	6,784,000
Net accumulated depreciation	82,826,000	84,073,000
Unamortized debt issuance expense	1,163,000	648,000
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 $769 ($794 in 2016)	0	0
Income tax receivable	(69,084,000)	(64,469,000)
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,705 and $7,975, respectively ($24,826 and $6,784, respectively, in 2016)	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	(21,965,000)	(38,664,000)
Investment in subsidiaries	(1,130,571,000)	(1,068,390,000)
Intercompany receivables	(135,586,000)	(75,820,000)
Total assets	(1,469,764,000)	(1,359,901,000)
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	(69,084,000)	(64,469,000)
Senior secured notes, net of debt issuance costs of $1,163 ($648 in 2016)	0	0
Subordinated indebtedness	(112,558,000)	(112,558,000)
Deferred tax liabilities, net	(21,965,000)	(38,664,000)
Intercompany payables	(135,586,000)	(75,820,000)
Total liabilities	(339,193,000)	(291,511,000)
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	(1,130,571,000)	(1,068,390,000)
Noncontrolling interest	0	0
Total stockholders' equity	(1,130,571,000)	(1,068,390,000)
Total liabilities and stockholders' equity	(1,469,764,000)	(1,359,901,000)
Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	3,716,000	10,284,000	2,586,000	1,557,000
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 $769 ($794 in 2016)	0	0
Income tax receivable	26,025,000	28,289,000
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	1,386,000	23,227,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,705 and $7,975, respectively ($24,826 and $6,784, respectively, in 2016)	0	0
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	20,221,000	21,963,000
Subordinated loan receivable	112,558,000	112,558,000
Intangible assets	0	0
Goodwill	0	0
Other assets	2,573,000	2,598,000
Deferred tax assets	0	309,000
Investment in subsidiaries	507,747,000	483,623,000
Intercompany receivables	83,437,000	37,914,000
Total assets	757,663,000	720,765,000
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	33,994,000	34,920,000
Income tax payable	22,189,000	22,189,000
Senior secured notes, net of debt issuance costs of $1,163 ($648 in 2016)	0	0
Subordinated indebtedness	0	0
Deferred tax liabilities, net	17,000	7,000
Intercompany payables	62,163,000	62,205,000
Total liabilities	118,363,000	119,321,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	639,300,000	601,444,000
Noncontrolling interest	0	0
Total stockholders' equity	639,300,000	601,444,000
Total liabilities and stockholders' equity	757,663,000	720,765,000
Non-Guarantor Subsidiaries
ASSETS
Cash and cash equivalents	36,996,000	54,400,000	59,871,000	61,811,000
Deposits with clearing organizations	42,222,000	38,185,000
Receivable from brokers, dealers and clearing organizations	187,115,000	214,934,000
Receivable from customers, net of allowance for credit losses of $769 ($794 in 2016)	848,226,000	847,386,000
Income tax receivable	0	0
Securities purchased under agreements to resell	658,000	24,006,000
Securities owned, at fair value	925,211,000	683,881,000
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,705 and $7,975, respectively ($24,826 and $6,784, respectively, in 2016)	40,520,000	30,099,000
Office facilities, net of accumulated depreciation of $104,065 ($97,118 in 2013)	6,966,000	5,270,000
Subordinated loan receivable	0	0
Intangible assets	31,700,000	31,700,000
Goodwill	137,889,000	137,889,000
Other assets	142,604,000	104,992,000
Deferred tax assets	18,463,000	37,961,000
Investment in subsidiaries	0	0
Intercompany receivables	0	0
Total assets	2,418,570,000	2,210,703,000
Liabilities
Drafts payable	42,412,000	39,228,000
Bank call loans	118,300,000	145,800,000
Payable to brokers, dealers and clearing organizations	211,483,000	221,389,000
Payable to customers	385,907,000	449,946,000
Securities sold under agreements to repurchase	586,478,000	378,084,000
Securities sold but not yet purchased, at fair value	94,486,000	85,050,000
Accrued compensation	173,116,000	145,053,000
Accounts payable and other liabilities	51,280,000	58,769,000
Income tax payable	44,455,000	39,840,000
Senior secured notes, net of debt issuance costs of $1,163 ($648 in 2016)	0	0
Subordinated indebtedness	112,558,000	112,558,000
Deferred tax liabilities, net	33,040,000	51,794,000
Intercompany payables	73,423,000	13,615,000
Total liabilities	1,926,938,000	1,741,126,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	491,271,000	466,946,000
Noncontrolling interest	361,000	2,631,000
Total stockholders' equity	491,632,000	469,577,000
Total liabilities and stockholders' equity	2,418,570,000	2,210,703,000
Parent
ASSETS
Cash and cash equivalents	7,442,000	229,000	907,000	$ 439,000
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 $769 ($794 in 2016)	0	0
Income tax receivable	45,998,000	41,996,000
Securities purchased under agreements to resell	0	0
Securities owned, at fair value	0	0
Notes receivable, net of accumulated amortization and allowance for uncollectibles of $24,705 and $7,975, respectively ($24,826 and $6,784, respectively, in 2016)	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	133,000	71,000
Deferred tax assets	3,502,000	394,000
Investment in subsidiaries	622,824,000	584,767,000
Intercompany receivables	52,149,000	37,906,000
Total assets	732,048,000	665,363,000
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	7,221,000	2,868,000
Income tax payable	2,440,000	2,440,000
Senior secured notes, net of debt issuance costs of $1,163 ($648 in 2016)	198,837,000	149,352,000
Subordinated indebtedness	0	0
Deferred tax liabilities, net	0	0
Intercompany payables	0	0
Total liabilities	208,498,000	154,660,000
Stockholders' equity
Stockholders' equity attributable to Oppenheimer Holdings Inc.	523,550,000	510,703,000	$ 518,058,000
Noncontrolling interest	0	0
Total stockholders' equity	523,550,000	510,703,000
Total liabilities and stockholders' equity	$ 732,048,000	$ 665,363,000