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Derivative Financial Instruments (Narrative) (Details) € in Thousands | Sep. 30, 2018 EUR (€) | Sep. 30, 2018 USD ($) | Dec. 31, 2017 EUR (€) | Dec. 31, 2017 USD ($)
--- | --- | --- | --- | ---
TBA And Other Forward Agency MBS [Member] | Short [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount |  | $ 1,304,554,000 |  | $ 1,029,844,000
TBA And Other Forward Agency MBS [Member] | Long [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount |  | 1,304,554,000 |  | 1,029,844,000
Foreign currency forward contracts [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount | € | € 1,000 |  | € 1,000 |
EuroDollar futures contracts [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative, Notional Amount |  | 0 |  | 0
Other Extended Settlement Trades [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Forward purchase commitment |  | 5,444,000 |  | 28,146,000
Forward sale commitment |  | $ 12,500,000 |  | $ 11,500,000

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- Definition Account transaction between trade date and settlement date as a derivative as either a forward purchase commitment or a forward sale commitment due to financial instrument not settling within regular time frame. + References No definition available. + Details
Name: | cohn_ForwardPurchaseCommitment
Namespace Prefix: | cohn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Account transaction between trade date and settlement date as a derivative as either a forward purchase commitment or a forward sale commitment due to financial instrument not settling within regular time frame. + References No definition available. + Details
Name: | cohn_ForwardSaleCommitment
Namespace Prefix: | cohn_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
Name: | invest_DerivativeNotionalAmount
Namespace Prefix: | invest_
Data Type: | xbrli:monetaryItemType
Balance Type: | na
Period Type: | instant

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

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=cohn_TbaAndOtherForwardAgencyMbsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PositionAxis=us-gaap_ShortMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_PositionAxis=us-gaap_LongMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeForwardMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_EurodollarFutureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |