Loans And Allowance For Credit Losses (Total Impaired Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Receivables [Abstract]
Non-accrual loans	$ 40,270	$ 10,220
Accruing Restructured Loans		71,512
Impaired Financing Receivable, Recorded Investment, Total		$ 81,732

Loans classified as troubled debt restructurings but are not on non-accrual status. Includes (1) loans renewed at interest rates judged not to be market rates for new debt with similar risk, and as a result were classified as troubled debt restructurings under ASC 310-40 but continue to perform in accordance with their modified terms, (2) certain credit card loans under various debt management and assistance programs, and (3) loans classified as troubled debt restructurings because they had not been reaffirmed by the borrower in bankruptcy proceedings but for which the borrowers continue to make payments.

No definition available.

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Namespace Prefix:	cbsh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant