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Financial Instruments - Quantitative Information about Level 3 Fair Value Measurements (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012
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Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
Securities purchases against purchase offers and legal settlement and proprietary position | $ 79.8
Auction Rate Securities [Member] |
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items] |
Commitment to purchase ARS from clients through 2016 | $ 40.5

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- Definition Securities committed to purchase amount. + References No definition available. + Details
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Namespace Prefix: | opy_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Securities purchases against purchase offers and legal settlement and proprietary position. + References No definition available. + Details
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Namespace Prefix: | opy_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration