Parent Company Condensed Financial Statements (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments in marketable common and preferred stock	$ 2,585,000	$ 48,838,000
Non-agency mortgage-backed securities	1,047,716,000	905,793,000
Equity securities redeemed for cash due to a third-party merger	39,900,000
Commerce Bancshares, Inc. (Parent) [Member]
Note receivable due from bank subsidiary	50,000,000	50,000,000
Parent's line of credit facility with the Bank, maximum borrowing capacity	20,000,000
Parent line of credit with Bank, borrowings during period	0	$ 0	$ 0
Investments in marketable common and preferred stock	2,500,000
Equity Securities without Readily Determinable Fair Value, Amount	720,000
Non-agency mortgage-backed securities	2,600,000
Equity securities redeemed for cash due to a third-party merger	$ 39,900,000