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.

No definition available.

+ Details

Name:	cohn_ForwardPurchaseCommitment
Namespace Prefix:	cohn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

+ References

No definition available.

+ Details

Name:	cohn_ForwardSaleCommitment
Namespace Prefix:	cohn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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

- 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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

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