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Parent Company Condensed Financial Statements (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Non-agency mortgage-backed securities $ 361,074,000 $ 809,782,000    
Investments in marketable common and preferred stock 2,966,000 2,929,000    
Commerce Bancshares, Inc. (Parent) [Member]        
Note receivable due from bank subsidiary 50,000,000 50,000,000   $ 50,000,000.0
Parent's line of credit facility with the Bank, maximum borrowing capacity 20,000,000.0      
Parent line of credit with Bank, borrowings during period 0 $ 0 $ 0  
Non-agency mortgage-backed securities 4,800,000      
Investments in marketable common and preferred stock 2,800,000      
Equity Securities without Readily Determinable Fair Value, Amount $ 300,000