Common Stock (Summary Of Components Used To Calculate Basic And Diluted Income Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Commerce Bancshares, Inc.	$ 132,448	$ 109,242	$ 224,168	$ 314,351
Less Preferred Stock Dividends	7,466	2,250	11,966	6,750
Net Income (Loss) Available to Common Stockholders	$ 124,982	$ 106,992	$ 212,202	$ 307,601
Basic income per common share	$ 1.12	$ 0.93	$ 1.90	$ 2.65
Diluted income per common share	$ 1.11	$ 0.93	$ 1.89	$ 2.65
Basic Income Per Common Share [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income attributable to Commerce Bancshares, Inc.	$ 132,448	$ 109,242	$ 224,168	$ 314,351
Less Preferred Stock Dividends	7,466	2,250	11,966	6,750
Net Income (Loss) Available to Common Stockholders	124,982	106,992	212,202	307,601
Less income allocated to nonvested restricted stockholders	1,173	1,040	1,996	2,996
Net income available to common stockholders	$ 123,809	$ 105,952	$ 210,206	$ 304,605
Weighted average common shares outstanding	110,720	112,982	110,848	114,475
Basic income per common share	$ 1.12	$ 0.93	$ 1.90	$ 2.65
Diluted Income Per Common Share [Member]
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net Income (Loss) Available to Common Stockholders, Diluted	$ 124,982	$ 106,992	$ 212,202	$ 307,601
Less income allocated to nonvested restricted stockholders	1,170	1,039	1,993	2,991
Net income available to common stockholders	$ 123,812	$ 105,953	$ 210,209	$ 304,610
Weighted average common shares outstanding	110,720	112,982	110,848	114,475
Net effect of the assumed exercise of stock-based awards - based on the treasury stock method using the average market price for the respective periods	179	267	209	284
Weighted average diluted common shares outstanding	110,899	113,249	111,057	114,759
Diluted income per common share	$ 1.11	$ 0.93	$ 1.89	$ 2.65