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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Trust Fees) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Trust Fees [Line Items]
Trust fees	$ 160,637	$ 155,628	$ 147,964
Private Client [Member]
Trust Fees [Line Items]
Trust fees	123,941	118,832	111,533
Institutional [Member]
Trust Fees [Line Items]
Trust fees	30,544	29,468	29,241
Other Trust Fees [Member]
Trust Fees [Line Items]
Trust fees	$ 6,152	$ 7,328	$ 7,190

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- Definition

[Line Items] for Disclosure of information of trust fees earned from contracts with customers

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No definition available.

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Namespace Prefix:	cbsh_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

Trust fee revenue

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No definition available.

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Name:	cbsh_Trustfeerevenue
Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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Name:	cbsh_TrustFeeProductandServicesAxis=cbsh_PrivateClientMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	cbsh_TrustFeeProductandServicesAxis=cbsh_InstitutionalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	cbsh_TrustFeeProductandServicesAxis=cbsh_OtherTrustFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: