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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Bank Card Transaction Fees) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	$ 151,797	$ 167,879	$ 171,576
Debit Card [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	37,644	40,025	39,738
Debit Card [Member] | Fee income [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	39,862	42,106	41,522
Debit Card [Member] | Network Charges Expense [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	(2,218)	(2,081)	(1,784)
Credit Card [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	13,393	14,177	12,965
Credit Card [Member] | Fee income [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	24,921	27,416	26,799
Credit Card [Member] | Network Charges and Rewards Expense [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	(11,528)	(13,239)	(13,834)
Corporate Card Fees [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	82,374	94,388	99,640
Corporate Card Fees [Member] | Fee income [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	179,251	196,984	199,651
Corporate Card Fees [Member] | Network Charges and Rewards Expense [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	(96,877)	(102,596)	(100,011)
Merchant Fees [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	18,386	19,289	19,233
Merchant Fees [Member] | Fee income [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	29,660	31,517	30,241
Merchant Fees [Member] | Network Charges Expense [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	(3,159)	(3,449)	(3,177)
Merchant Fees [Member] | Fees to Cardholder Banks [Member]
Bank Card Transaction Fees [Line Items]
Bank card transaction fees	$ (8,115)	$ (8,779)	$ (7,831)

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- Definition

Fees earned from the issuance of debt and credit cards to the Company's retail and commercial banking customers who use the cards to purchase goods and services from merchants through an electronic payment system.

+ References

No definition available.

+ Details

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

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- Definition

[Line Items] for Disclosure of information about bank card transaction fees earned from contracts with customers.

+ References

No definition available.

+ Details

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

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- Details

Name:	cbsh_BankCardProductandServicesAxis=us-gaap_DebitCardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	cbsh_BankCardTransactionsAxis=cbsh_FeeincomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cbsh_BankCardTransactionsAxis=cbsh_NetworkChargesExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cbsh_BankCardProductandServicesAxis=us-gaap_CreditCardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cbsh_BankCardTransactionsAxis=cbsh_NetworkChargesandRewardsExpenseMember
Namespace Prefix:
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Balance Type:
Period Type:

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- Details

Name:	cbsh_BankCardProductandServicesAxis=cbsh_CorporateCardFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	cbsh_BankCardProductandServicesAxis=cbsh_MerchantFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	cbsh_BankCardTransactionsAxis=cbsh_FeestoCardholderBanksMember
Namespace Prefix:
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Balance Type:
Period Type: