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New Accounting Pronoucements - Additional Information (Narrative) (Details) $ in Millions | Sep. 30, 2018 USD ($) location
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New Accounting Pronouncements [Line Items] |
Number of locations under operating leases | location | 100
Minimum [Member] |
New Accounting Pronouncements [Line Items] |
Operating Lease, Right-of-Use Asset | $ 170.0
Maximum [Member] |
New Accounting Pronouncements [Line Items] |
Operating Lease, Right-of-Use Asset | $ 190.0

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- Definition Number of locations under operating leases + References No definition available. + Details
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Namespace Prefix: | opy_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Summary Of Significant Accounting Policies [Line Items] + References No definition available. + Details
Name: | opy_SummaryOfSignificantAccountingPoliciesLineItems
Namespace Prefix: | opy_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of lessee's right to use underlying asset under operating lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 842 -SubTopic 20 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77888419&loc=SL77918627-209977 + Details
Name: | us-gaap_OperatingLeaseRightOfUseAsset
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |