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Note 12 - Investments in Equity Method Affiliates (Details Textual) - SPAC Sponsor Entities [Member] | 12 Months Ended
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Dec. 31, 2020 shares
Equity Method Investment, Shares Purchased (in shares) | 765,000
Equity Method Investment, Shares Received (in shares) | 1,074,000

X
- Definition The number of shares received upon purchase of investment accounted for under the equity method of accounting. + References No definition available. + Details
Name: | cohn_EquityMethodInvestmentSharesPurchased
Namespace Prefix: | cohn_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents shares received from equity method method investment. + References No definition available. + Details
Name: | cohn_EquityMethodInvestmentSharesReceived
Namespace Prefix: | cohn_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis=cohn_SPACSponsorEntitiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |