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Fair Value Measurements - Investments in Company-Sponsored Funds (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Investment Holdings [Line Items] |  |  |
Fair Value | $ 8,827,000 us-gaap_InvestmentsFairValueDisclosure |  |
Unfunded Commitments | 1,251,000 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments |  |
Hedge Funds |  |  |
Investment Holdings [Line Items] |  |  |
Fair Value | 2,159,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember | [1] |
Unfunded Commitments | 0 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_HedgeFundsMember | [1] |
Redemption Frequency | Quarterly - Annually | [1] |
Redemption Notice Period, Minimum | 30 days | [1] |
Redemption Notice Period, Maximum | 120 days | [1] |
Private Equity Funds |  |  |
Investment Holdings [Line Items] |  |  |
Fair Value | 6,668,000 us-gaap_InvestmentsFairValueDisclosure / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember | [2] |
Unfunded Commitments | 1,251,000 us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnfundedCommittments / us-gaap_InvestmentTypeAxis = us-gaap_PrivateEquityFundsMember | [2] |
Auction rate securities |  |  |
Investment Holdings [Line Items] |  |  |
Assets and liabilities transferred from level one and level two | $ 0 opy_AssetsAndLiabilitiesTransferredFromLevelOneToLevelTwo / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AuctionRateSecuritiesMember |  | $ 0 opy_AssetsAndLiabilitiesTransferredFromLevelOneToLevelTwo / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AuctionRateSecuritiesMember
[1] | Includes investments in hedge funds and hedge fund of funds that pursue long/short, event-driven, and activist strategies. Each hedge fund has various restrictions regarding redemption; no investment is locked-up for a period greater than one year.
[2] | Includes private equity funds and private equity fund of funds with a focus on diversified portfolios, real estate and global natural resources. Due to the illiquid nature of these funds, investors are not permitted to make withdrawals without consent of the general partner. The lock-up period of the private equity funds can extend to 10 years.

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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph a, f, g -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24584-111560 + Details
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