Fair Value Measurements - Narrative (Details) - USD ($)	9 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction Rate Securities Committed To Purchase related to settlements with regulators	$ 5,000,000
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	2,169,000	$ 2,024,000
Derivative Liability, Notional Amount	3,514,485,000	4,756,631,000
Principal	124,523,000
Amount Of Auction Rate Securities Firm Purchased And Hold Net of Redemptions	109,000,000
Auction Rate Securities Committed To Purchase From Clients under Legal Settlements and Awards	10,500,000
Securities purchased under agreements to resell, fair value		$ 24,006,000
Valuation adjustment (unrealized loss) for ARS	$ (2,492,000)
Forward or delayed delivery of the underlying instrument with settlement (in days)	180 days
Fair Value Of Level Three Assets	$ 122,031,000
Auction Rate Securities Owned [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Principal	109,000,000
Valuation adjustment (unrealized loss) for ARS	(2,216,000)
Fair Value Of Level Three Assets	106,784,000
Auction Rate Securities Purchase Commitment [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Principal	15,523,000
Valuation adjustment (unrealized loss) for ARS	(276,000)
Fair Value Of Level Three Assets	$ 15,247,000
Maximum [Member] \| LIBOR or Treasury Yield [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Auction market preferred securities variable interest rate earned	200.00%
Auction Rate Securities [Member] \| Fair Value, Valuation Scenario Two [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Change in discount rate	50.00%
Decrease in fair value of ARS	$ 2,300,000
Auction Rate Securities [Member] \| Fair Value, Valuation Scenario One [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Change in discount rate	25.00%
Decrease in fair value of ARS	$ 1,200,000
Auction Rate Securities [Member] \| Maximum [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commitment to purchase ARS period maximum	2020

Amount Of Auction Rate Securities Firm Purchased And Hold Net of Redemptions

No definition available.

Name:	opy_AmountOfAuctionRateSecuritiesFirmPurchasedAndHoldNetofRedemptions
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Auction Rate Securities Committed To Purchase From Clients under Legal Settlements and Awards

+ References

No definition available.

+ Details

Name:	opy_AuctionRateSecuritiesCommittedToPurchaseFromClientsunderLegalSettlementsandAwards
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Auction Rate Securities Committed To Purchase related to settlements with regulators

+ References

No definition available.

+ Details

Name:	opy_AuctionRateSecuritiesCommittedToPurchaserelatedtosettlementswithregulators
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of the level 3 assets used for the quantitative disclosure.

+ References

No definition available.

+ Details

Name:	opy_FairValueOfLevelThreeAssets
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair Value Of Securities Reverse Repurchase Agreement

+ References

No definition available.

+ Details

Name:	opy_FairValueOfSecuritiesReverseRepurchaseAgreement
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount represents the principal amount of the level 3 assets used for the quantitative disclosure.

+ References

No definition available.

+ Details

Name:	opy_LevelThreePrincipalAmount
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Maximum settlement period in case of delayed delivery of underlying instrument.

+ References

No definition available.

+ Details

Name:	opy_MaximumSettlementPeriodInCaseOfDelayedDeliveryOfUnderlyingInstrument
Namespace Prefix:	opy_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Securities committed to purchase period.

+ References

No definition available.

+ Details

Name:	opy_SecuritiesCommittedToPurchasePeriod
Namespace Prefix:	opy_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Sensitivity analysis of fair value of interests continued to be held by transferor servicing assets or liabilities change in discount rate.

+ References

No definition available.

+ Details

Name:	opy_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesChangeInDiscountRate
Namespace Prefix:	opy_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Valuation adjustment for the level 3 assets used for quantitative disclosure.

+ References

No definition available.

+ Details

Name:	opy_ValuationAdjustmentForLevelThreeAssets
Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The interest (or dividend) rates earned on action market preferred securities (on an annual basis or APY) for the period reported.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

+ Details

Name:	us-gaap_AuctionMarketPreferredSecuritiesStockSeriesVariableInterestRateEarned
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value, after offset of derivative asset, of financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=d3e41678-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226006-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountNotOffsetAgainstCollateral
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5580258-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeLiabilityNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The hypothetical financial impact of a specified adverse percentage change in discount rate on the fair value of transferor's interests in transferred financial assets (including any servicing assets or servicing liabilities) as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

+ Details

Name:	us-gaap_SensitivityAnalysisOfFairValueOfInterestsContinuedToBeHeldByTransferorServicingAssetsOrLiabilitiesImpactOfOtherThan10Or20PercentAdverseChangeInDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis=opy_AuctionRateSecuritiesOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SecurityOwnedAndSoldNotYetPurchasedAtFairValueAxis=opy_AuctionRateSecuritiesPurchaseCommitmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=opy_LIBORorTreasuryYieldMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AuctionRateSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=opy_FairValueValuationScenarioTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=opy_FairValueValuationScenarioOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: