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Commitments and Contingencies Narrative (Details)	9 Months Ended
	Sep. 30, 2018 USD ($)
Loss Contingencies [Line Items]
Eligible Investor Subject To Future Buyback Principal Value	$ 7,700,000
Minimum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Range of Possible Loss, Portion Not Accrued	0
Eligible Investor Subject To Future Buyback Potential Additional Losses Related To Valuation Adjustments	0
Maximum [Member]
Loss Contingencies [Line Items]
Loss Contingency, Range of Possible Loss, Portion Not Accrued	30,000,000
Auction Rate Securities [Member]
Loss Contingencies [Line Items]
Amount Of Auction Rate Securities Firm Purchased And Hold	40,800,000
Auction Rate Securities Purchase Commitment [Member]
Loss Contingencies [Line Items]
Auction Rate Securities Committed To Purchase From Client	$ 7,200,000

X

- Definition

The total amount of ARS the firm purchased and hold.

+ References

No definition available.

+ Details

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Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The amount of ARS the firm committed to purchase from clients.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Eligible investor subject to future buyback potential additional losses related to valuation adjustments.

+ References

No definition available.

+ Details

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Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
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X

- Definition

Eligible investor subject to future buyback principal value.

+ References

No definition available.

+ Details

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Namespace Prefix:	opy_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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X

- 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

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X

- Definition

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=99385795&loc=d3e12631-108344

+ Details

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X

- Details

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X

- Details

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- Details

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X

- Details

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