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Financial instruments - Quantitative Information about Level 3 Fair Value Measurements (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Securities purchases against purchase offers and legal settlement and proprietary position	$ 77.1
Auction Rate Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Commitment to purchase ARS from clients through 2016	$ 38.3

X

- Definition

Securities committed to purchase amount.

+ References

No definition available.

+ Details

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

X

- 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

X

- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration