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Revenue from Contracts with Customers Revenue from Contracts with Customers (Schedule of Deposit Account Charges and Other Fees) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deposit Account Charges and Other Fees [Line Items]
Deposit account charges and other fees	$ 93,227	$ 95,983	$ 94,517
Corporate Cash Management [Member]
Deposit Account Charges and Other Fees [Line Items]
Deposit account charges and other fees	46,762	41,442	38,468
Overdraft and Return Item [Member]
Deposit Account Charges and Other Fees [Line Items]
Deposit account charges and other fees	22,951	30,596	31,468
Other Deposit Account Charges [Member]
Deposit Account Charges and Other Fees [Line Items]
Deposit account charges and other fees	$ 23,514	$ 23,945	$ 24,581

X

- Definition

[Line Items] for Disclosure of information for deposit account charges and other fees from contracts with customers.

+ References

No definition available.

+ Details

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

X

- Definition

Deposit fee revenue

+ References

No definition available.

+ Details

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

X

- Details

Name:	cbsh_DepositaccountfeeproductandservicesAxis=cbsh_CorporateCashManagementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cbsh_DepositaccountfeeproductandservicesAxis=cbsh_OverdraftandReturnItemMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cbsh_DepositaccountfeeproductandservicesAxis=cbsh_OtherDepositAccountChargesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: