Note 6 - Receivables from and Payables to Brokers, Dealers, and Clearing Agencies - Receivables from and Payables to from Brokers, Dealers and Clearing Agencies (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deposits with clearing organizations	$ 250	$ 250
Unsettled regular way trades, net	2,961	12,170
Receivable from clearing organizations	49,706	83,712
Receivables from brokers, dealers, and clearing agencies	52,917	96,132
Margin payable	156,678	208,441
Due to clearing agent		32,820
Payables to brokers, dealers, and clearing agencies	$ 156,678	$ 241,261

The amount of receivables from unsettled regular way trades.

No definition available.

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

- Definition

Amount payable to other broker-dealer and clearing organization, including, but not limited to, security failed-to-receive, deposit received for security loaned, open transaction, and floor-brokerage payable.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

+ Details

Name:	srt_PayablesToBrokerDealersAndClearingOrganizations
Namespace Prefix:	srt_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount payable to broker-dealers or clearing organizations for executing or clearing trades or orders.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=68072869&loc=d3e41256-110953

+ Details

Name:	us-gaap_CommissionsPayableToBrokerDealersAndClearingOrganizations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of securities on deposit with clearing organizations and other entities which provide services necessary to operations of the Company. This item would include, for instance, securities advanced as refundable collateral for securities borrowed.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DepositsWithClearingOrganizationsAndOthersSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of other payables due to broker-dealers or clearing organizations.

+ References

No definition available.

+ Details

Name:	us-gaap_OtherPayablesToBrokerDealersAndClearingOrganizations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount receivable from broker-dealers and clearing organizations, including, but not limited to, securities failed-to-deliver, certain deposits for securities borrowed, open transactions, good faith and margin deposits, commissions and floor brokerage receivables.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=35710130&loc=d3e41645-110958

+ Details

Name:	us-gaap_ReceivablesFromBrokersDealersAndClearingOrganizations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of right to receive cash and securities on deposit with clearing organizations. Clearing organizations perform post trade processing and trade comparisons among numerous broker-dealers and act as settlement agents between buying and selling broker-dealers.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 310
-URI http://asc.fasb.org/subtopic&trid=2176284

+ Details

Name:	us-gaap_ReceivablesFromClearingOrganizations
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant