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Note 9 - Fair Value Disclosures (Details Textual) - Other Investment [Member] - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Fair Value, Option, Changes in Fair Value, Gain (Loss) | $ 43,413 | $ 1,080 | $ 2,616
Other Investment Sold, Not yet Purchased, Gain (Loss) | $ 1,063 | $ 0 | $ 0

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- Definition Represents realized and unrealized gain (loss) for other investment sold not yet purchased. + References No definition available. + Details
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Namespace Prefix: | cohn_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |