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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 |  |  |

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- Definition The fair value as of the balance sheet date of firm holdings in non-agency mortgage-backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the non-agency mortgage-backed security). + References No definition available. + Details
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