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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Consumer Brokerage Services) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Consumer Brokerage Services [Line Items] |  |  |
Brokerage Commissions Revenue | $ 15,095 | $ 15,804 | $ 15,807
Commission Income [Member] |  |  |
Consumer Brokerage Services [Line Items] |  |  |
Brokerage Commissions Revenue | 8,002 | 9,071 | 8,956
Managed Account Services [Member] |  |  |
Consumer Brokerage Services [Line Items] |  |  |
Brokerage Commissions Revenue | $ 7,093 | $ 6,733 | $ 6,851

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- Definition [Line Items] for Disclosure of information of fees earned from consumer brokerage services from contracts with customers. + References No definition available. + Details
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Namespace Prefix: | cbsh_
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- Definition Amount of commission revenue from buying and selling securities on behalf of customers. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 940 -SubTopic 20 -Section 05 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=68070981&loc=d3e40544-110947 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(a),(b)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 + Details
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Balance Type: | credit
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- Details
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- Details
Name: | cbsh_ConsumerBrokerageProductandServicesAxis=cbsh_ManagedAccountServicesMember
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Data Type: | na
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Period Type: |